Your mail hasn't disappeared, TB has just lost track of the accounts, probably due to a corrupted prefs file:

https://support.mozilla.org/en-US/questions/1360261

Thank you for that prompt and helpful reply!

Yes, I knew it was there, but was immensely frustrated that I could not view it in Thunderbird. However, once I clicked through all the links in your various messages to get to the actual instructions and apply them, it did the trick. For the benefit of anyone else with this problem, here is an edited version of the instructions:

- Open the Thunderbird profile folder and search for files named "prefs-1.js", "prefs-2.js", etc.. - Open the most recent copy in a text editor and see if it has the correct account data, and if so, rename "prefs.js" as "prefs.js.bak" (as a back-up), then rename the most recent "prefs-#.js" (# being a number) file that looks correct as prefs.js. - Restart Thunderbird and you should have everything as it was on the date of that last "prefs-#.js" file.

"If you do a search in the c:\users\<yourid>\appdata\roaming\thunderbird\profiles folder, you may find abook.sqlite and history.sqlite. These are the technical filenames of the addresssbook and the collected addressbook. You can import them with tools>import>importfromafile. This sometimes happens, that thunderbird creates a new profile and some content is left behind. No, this does not happen to everyone and is likely a glitch wherein an update was interrupted by another program, such as an antivirus product."

This might help. search for "abook.sqlite". Found it in a file, C:/windows.old. look for last oldest.