It may be there was an update to Thunderbird and, if you use an anti-virus product, it may have interrupted the update. In such cases, Thunderbird creates a new profile, but it does not delete the old one. So, if you browse in the area of c:\users\<yourid>\appdata\roaming\thunderbird\profiles, you may see the older profile, complete with your accounts and folders. If the profile isn't corrupted you can recover it.

Thanks David! Update to my problem, I realize that my old Thunderbird was set up as POP. When asked to 'update' I mistakenly signed back as IMAP. I don't quite understand how all this works, but that was when I lost all old emails and folders.

I changed the account back to POP (per Thunderbird instructions) but still don't see old emails or folders.

I think that when the original problem started, Thunderbird created a new Profile. I cannot find an old Profile, and unfortunately, I did not have back up :(

If there is an old profile, it would be a folder at c:\users\<yourid>\appdata\roaming\thunderbird\profiles. In the folder would be a Mail folder and a POP account would be there. If such does not exist, then it's lost.