I updated to 64bit and lost 3 years of emails back to 2016 for current folders.
I updated Thunderbird to 64bit. When I ran it, I only had emails up to 2016. In some cases, if the folder was a new one then the emails to 2019 are there. Many folders are missing. Also, my address book is the same way. Two days ago before I did the upgrade, AT&T changed their servers from att.yahoo to att but I don't think that damaged anything since this was before the Thunderbird upgrade.
Modified
Chosen solution
go to the about:profiles entry in the troubleshooting information (Help menu)
Select the profile listed that is not your current default profile and make it the default and restart Thunderbird.
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PS. I found a backup profile from 6 months ago. I moved the latest profile to a temp directory (just in case) and put the backup profile in. Same problem.
Chosen Solution
go to the about:profiles entry in the troubleshooting information (Help menu)
Select the profile listed that is not your current default profile and make it the default and restart Thunderbird.