I was able to get back to Thunderbird 91.10.0 by reinstalling the older version, restoring from my profile backup, and then starting Thunderbird from the the Windows 10 RUN dialog as follows:

thunderbird.exe -p

That brought up a menu of profiles. I only had two, and I was able to select my old profile. Then I had all my emails, folders, and accounts restored. The sign-ons worked too. I was incredibly relieved.

While I was on TB 102.0, I was NOT able to create an account for my current email profile. The authentication failed. I was also not able to re-create my GigaNews usenet account, nor for my grc.com private usenet account.

I don't know if this affects the problem, but I only use POP3 for my email and usenet accounts. I do not want to use IMAP because I do not want my old emails stored in the cloud.

I had this problem just now, too. Thunderbird surprised me with the upgrade without warning. Only my linked gmail account carried over. The two email addresses on my personal domain did not; that is to say there was a heading and settings but no emails or folders from the imap servers.

I backed-up my profile to a zip and moved it out of the default location. Then I chose to Import my account again. It prompted me that any duplicates would be overwritten which would've been fine since I'd already backed up.

It seems to have worked just now. All the mail is there, send and receive works...and I'm still on the latest version.

I stumbled upon a quicker solution that worked for my own case: simply close then re-open Thunderbird 102.

After upgrading to Thunderbird 102, my personal domain email account disappeared entirely without even an empty stub. But in the course of preparing to take steps suggested by either arnold.trembley or andrew191, I happened to re-open Thunderbird 102 and all my email accounts just re-appeared! I didn't have to temporarily roll back to an earlier version of Thunderbird, and I didn't have to re-import my profile.

My original problem was also affected by a couple of known bugs related to downloading the group names list for NNTP servers (such as giganews.com). But knowing how to backup and restore my Thunderbird profile was very helpful. Version 102.3.2 was the first version with all the bug fixes I needed. I also had an old .xpi in my profile that caused conflicts, so I had to build a new profile and then copy over all my email folders into. My Thunderbird is fully recovered now, and I am on version 102.4.0.