Update:

I have 5 email accounts in my Thunderbird app. All 5 exhibit the same behavior...that is, no emails are listed in the inbox even though I know there are emails that should be there, nor in any of the folders for each account. On one account (Hotmail) There is an unread flag showing 60 unread even though none are visile.

Was wondering if I have exceeded the mailbox size and if installing the 64bit version would help. Would I lose my existing emails and folders?

Hint: I tried to backup my thunderbird subdirectory to a 32gb flash drive and it ran out of space. I got an error saying that my mail file was over 5gb.

If I need to upgrade to the 64bit version, what would be the proper steps to ensure not losing anymore data?

Thanks. This is getting scary.

Update: I tried to repair the folder index (.msf file) but the location is blank in the Folder Properties fill-in page and I can't find a way to point it to the proper subdirectory. Can anyone help? My whole life is in those folders. Thanks.

Problem only partially solved...

I can see and read the emails in TB now but cannot delete or move them. Also unable to retrieve, repair, or compact.

I uninstalled and reinstalled TB. Tried to do so on two different drives, no help. Can't add a new profile. Getting this error..

                    An unexpected error has prevented your changes from being saved.

Interesting that another user also reported having a similar error beginning on the same date, Dec. 12, 2023. Any ideas?

Thanks much.

Check that your profile directory is writable. Help > Troubleshoot Information > show profile directory

Also, if your accounts are imap then you will not lose any mail by creating a new profile. https://support.mozilla.org/en-US/kb/using-multiple-profiles.