To diagnose problems with Thunderbird, try one of the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

I tried both; add-ons disabled and restarted windows 7 in safe mode, neither made any difference. I am getting a message: Sending of password for (email) did not succeed. Mail server mail.charter.net responded: invalid user name or password. I have tried old passwords and made up new passwords and nothing works. Spectrum says that POP3 will still work, that is what I have always used, but I am wondering if I have to change to IMAP. Any ideas for settings or other ideas would be appreciated. Thanks, Glenn

I tried looking at your providers web site. https://www.spectrum.net/support/internet/spectrum-email-server-settings/ Under settings for charter they give the server names as. Charter.net

• Incoming Email Server: mobile.charter.net
• Outgoing Email Server: mobile.charter.net

Your error message says it is from mail.charter.net so perhaps you might adjust your account settings. Both incoming and in outgoing server (SMTP) to match spectrum's advice in this.,