I would have exited Thunderbird and deleted global-messages-db.sqlite to force a new one to be created after restarting Thunderbird, but renaming should have worked. However, when I say 'worked' this does not mean it would have got emails back. It is more likely to not show them up in the Global Search.

It sounds like you have a search function that is locating an out of date set of data.

When you have an email in the Thread Pane list which when selected shows blank contents, the 'Repair Folder' would be the fix to ensure the index is truelly reflecting what is really in the mbox file. Depending upon what is in the mbox file this may reveal emails if index was misaligned or remove the out of date headers to non existant emails. 'Repair Folder' does not delete emails, if they do not appear then they were already gone.

The 'Quick Filter Bar' has a search which is ideally suitable for searching the selected folder. .

The 'Mail Toolbar' has the 'Global search' text box option which searches throughout the accounts and allows for further filtering.

Classic Search: 'Menu icon' > 'Find' > 'Search Messages' has the option to search specific accounts or folders and also create a Search folder for results.

Windows Search can also search accounts and folders for emails if it has been enabled in this location. 'Menu icon' > 'Options' > Advanced' > 'General' tab Select checkbox: 'Allow Windows Search to search messages'

The Windows Search integration uses *.wdseml files, which are a partial copy (up to 49KB) of a message in the mail folder, stored in a .mozmsgs subdirectory in your profile folders. This type of search does not use the global-messages-db.sqlite.

Which makes me wonder, what 'Search' option are you using? Do you have any *.mozmsgs folders in your accounts profile folders?

Are you talking about a pop or imap mail account Inbox?

Thanks for your reply Toad-Hall. I do not have any *.mozmsgs folder on my PC. I have a POP account. When I searched I was using the search edit box near the top of the Thunderbird main window which can be focused with Ctrl K. But now when I search I no longer see emails in my inbox as I did before. I don't know when this changed; possible after renaming global-messages-db.sqlite. I think I have to accept that I've permanently lost all my emails which for my business is a problem. My last backup was about 6 weeks ago so that is better than nothing. How do I add the backup of inbox to my email? Could I just copy inbox to say inboxAA and would Thunderbird index it? I'll try that.

Yes, that works. I closed TB, copied the backup to inboxAA in the same folder as inbox, removed global-messages-db.sqlite and started TB again. After a delay I could read all the old emails up to the time they were backed up.