In order for us to better investigate this issue you need to add a little more non-personal information.

Please add the troubleshooting information to your post To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help)
• Choose Troubleshooting Information
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Please also add the result of these troubleshooting steps to your post

• Does Thunderbird work in TB Safe mode (see Thunderbird Safe Mode)?
• Do you use anti-virus and firewall software? What is the version?

Thunderbird did NOT work in TB Safe mode. Microsoft Security Essentials V 4.9.218.0 Malwarebytes Premium 3.1.2

Application Basics

   Name: Thunderbird
   Version: 52.9.1
   User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20180709124824
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account1:
     INCOMING: account1, , (pop3) inbound.att.net:995, SSL, passwordCleartext
     OUTGOING: , outbound.att.net:465, SSL, passwordCleartext, true

   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

 Crash Reports

 Extensions
   Lightning, 5.4.9.1, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   Theme Font & Size Changer, 56.0, true, {f69e22c7-bc50-414a-9269-0f5c344cd94c}

 Important Modified Preferences

   Name: Value

     accessibility.lastLoadDate: 1532017935
     browser.cache.disk.capacity: 358400
     browser.cache.disk.filesystem_reported: 1
     browser.cache.disk.smart_size_cached_value: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     dom.apps.reset-permissions: true
     extensions.lastAppVersion: 52.9.1
     font.minimum-size.x-western: 12
     font.name.monospace.el: Consolas
     font.name.monospace.x-cyrillic: Consolas
     font.name.monospace.x-unicode: Consolas
     font.name.monospace.x-western: Consolas
     font.name.sans-serif.el: Calibri
     font.name.sans-serif.x-cyrillic: Calibri
     font.name.sans-serif.x-unicode: Calibri
     font.name.serif.el: Cambria
     font.name.serif.x-cyrillic: Cambria
     font.name.serif.x-unicode: Cambria
     font.name.serif.x-western: Comic Sans MS
     font.size.fixed.el: 14
     font.size.fixed.x-cyrillic: 14
     font.size.fixed.x-unicode: 14
     font.size.fixed.x-western: 14
     font.size.variable.el: 17
     font.size.variable.x-cyrillic: 17
     font.size.variable.x-unicode: 17
     font.size.variable.x-western: 14
     gfx.crash-guard.d3d11layers.appVersion: 52.9.1
     gfx.crash-guard.d3d11layers.deviceID: 0x1381
     gfx.crash-guard.d3d11layers.driverVersion: 22.21.13.8205
     gfx.crash-guard.d3d11layers.feature-d2d: true
     gfx.crash-guard.d3d11layers.feature-d3d11: true
     gfx.crash-guard.glcontext.gfx.driver-init.direct3d11-angle: true
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle: true
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-force-d3d11: false
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-force-warp: false
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-try-d3d11: true
     gfx.crash-guard.status.d3d11layers: 2
     gfx.crash-guard.status.glcontext: 2
     gfx.direct2d.disabled: false
     gfx.direct3d.last_used_feature_level_idx: 0
     layers.acceleration.disabled: false
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: 0e336812-c8f5-4eab-8fb1-2bdd9cd0ebd
     media.gmp.storage.version.observed: 1
     network.cookie.cookieBehavior: 2
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1532017625
     places.history.enabled: false
     places.history.expiration.transient_current_max_pages: 122334
     plugin.importedState: true
     print.printer_Brother_HL-4040CN_series.print_bgcolor: false
     print.printer_Brother_HL-4040CN_series.print_bgimages: false
     print.printer_Brother_HL-4040CN_series.print_duplex: -437918235
     print.printer_Brother_HL-4040CN_series.print_edge_bottom: 0
     print.printer_Brother_HL-4040CN_series.print_edge_left: 0
     print.printer_Brother_HL-4040CN_series.print_edge_right: 0
     print.printer_Brother_HL-4040CN_series.print_edge_top: 0
     print.printer_Brother_HL-4040CN_series.print_evenpages: true
     print.printer_Brother_HL-4040CN_series.print_footercenter:
     print.printer_Brother_HL-4040CN_series.print_footerleft:
     print.printer_Brother_HL-4040CN_series.print_footerright:
     print.printer_Brother_HL-4040CN_series.print_headercenter:
     print.printer_Brother_HL-4040CN_series.print_headerleft:
     print.printer_Brother_HL-4040CN_series.print_headerright:
     print.printer_Brother_HL-4040CN_series.print_in_color: true
     print.printer_Brother_HL-4040CN_series.print_margin_bottom: 0.5
     print.printer_Brother_HL-4040CN_series.print_margin_left: 0.5
     print.printer_Brother_HL-4040CN_series.print_margin_right: 0.5
     print.printer_Brother_HL-4040CN_series.print_margin_top: 0.5
     print.printer_Brother_HL-4040CN_series.print_oddpages: true
     print.printer_Brother_HL-4040CN_series.print_orientation: 0
     print.printer_Brother_HL-4040CN_series.print_page_delay: 50
     print.printer_Brother_HL-4040CN_series.print_paper_data: 1
     print.printer_Brother_HL-4040CN_series.print_paper_height: -1.00
     print.printer_Brother_HL-4040CN_series.print_paper_name:
     print.printer_Brother_HL-4040CN_series.print_paper_size_unit: 0
     print.printer_Brother_HL-4040CN_series.print_paper_width: -1.00
     print.printer_Brother_HL-4040CN_series.print_resolution: 600
     print.printer_Brother_HL-4040CN_series.print_reversed: false
     print.printer_Brother_HL-4040CN_series.print_scaling: 1.00
     print.printer_Brother_HL-4040CN_series.print_shrink_to_fit: false
     print.printer_Brother_HL-4040CN_series.print_to_file: false
     print.printer_Brother_HL-4040CN_series.print_unwriteable_margin_bottom: 0
     print.printer_Brother_HL-4040CN_series.print_unwriteable_margin_left: 0
     print.printer_Brother_HL-4040CN_series.print_unwriteable_margin_right: 0
     print.printer_Brother_HL-4040CN_series.print_unwriteable_margin_top: 0
     privacy.donottrackheader.enabled: true
     security.sandbox.content.tempDirSuffix: {33eaac2f-d5ab-40eb-a6b1-ba6d2a23ed9f}

 Graphics

     GPU #1
     Description: NVIDIA GeForce GTX 750
     Vendor ID: 0x10de
     Device ID: 0x1381
     RAM: 1024
     Drivers: nvd3dumx,nvwgf2umx,nvwgf2umx nvd3dum,nvwgf2um,nvwgf2um
     Driver Version: 22.21.13.8205
     Driver Date: 5-1-2017

     Features
     Direct2D: true
     DirectWrite: true (6.2.9200.22164)
     WebGL Renderer: Google Inc. -- ANGLE (NVIDIA GeForce GTX 750 Direct3D11 vs_5_0 ps_5_0) -- OpenGL ES 2.0 (ANGLE 2.1.0.2a250c8a0e15)

     AzureCanvasBackend: direct2d 1.1
     AzureCanvasAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: direct2d 1.1

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.13.1
     4.13.1

     NSS
     3.28.6
     3.28.6

     NSS Util
     3.28.6
     3.28.6

     NSS SSL
     3.28.6
     3.28.6

     NSS S/MIME
     3.28.6
     3.28.6

"ATT secure mail key"? I don't know what you are referring. I have been using my password w/o any problems for years. Thunderbird has been working fine until yesterday so I don't think the problem is with my password.

wrangler53 said

"ATT secure mail key"? I don't know what you are referring. I have been using my password w/o any problems for years. Thunderbird has been working fine until yesterday so I don't think the problem is with my password.

ATT not longer support passwords. read the information in the link sfhowes gave you.

Well, today I was able to use Thunderbird again but I am at a loss as to explain why. I did find info on ATT.net support about Secure mail key. I attempted to follow their directions to create a key but the process did not appear to work. I got as far as entering a nickname for the key but when I clicked on the "Create Key" tab nothing happened. I did not see a key created nor was there an options for me to create a key.

When I tried using Thunderbird this A.M. I logged in using my same old password and I was able to receive my emails. There was an email from AT&T notifying me that my profile had been changed with a link to a page to contact if I had not made the change. Since I had not been able to enter a secure key, as far as I could tell, I clicked on the link. It was absolutely useless! It simply sent me to pages with general info about secure key! Really aggravating! I then logged in to my AT&T profile to see what had changed, I selected the "Secure Key" option and all I received was the same screen I had used to "Create Secure Key" (i.e., enter nickname, create key.) Totally useless! More aggravation!

Still have no idea what happened. Did AT&T create a secure key without informing me what it is? Why cannot I find the key if it was created? Did the secure key requirement actually have anything to do with why I could not use Thunderbird for two day?

I edited the last post so I could read it, but I have no answers to why anything does or does not work at ATT or Yahoo. They are to me a black hole which accepts reports that nothing useful ever comes out of.