Are you using POP or IMAP? Were the messages present when you shut down yesterday and missing when you logged in today? Any observation on what you think may have caused this? I'm no guru with Thunderbird, but maybe getting more information available will help others contribute to this.

IMAP. Everything was there in the afternoon. When I looked later that evening, the inbox was empty. Messages aren't in spam or trash. I shut down overnight, and it's no different this morning. I can get mail, but nothing older than yesterday shows in my inbox. My folders are ok, so I didn't lose everything, but anything I hadn't categorized yet is gone.

Check that you don't have a filter in place that shows only unread mail. You should have View/Threads set to All, and not have any Quick Filter Bar buttons selected.

I do have those settings. I'm getting all mail from today with no apparent issues. But everything that was in my inbox yesterday disappeared.

I suggest you contact your IMAP provider to see if they can restore the mail for you.

If you view the account in webmail, are the missing messages in the Inbox?

I looked, and no, they're gone from there as well.

What Anti-Virus program do you use? Do you allow an Anti-Virus program to scan Thunderbird files and folders or scan incoming.outgoing emails ? If yes, Advise is do not allow the Anti-virus to automatically fix, it must ask what to do and you must not tell it to fix.

It is best to not allow any Anti-Virus or Clean up file type of software any access to any Thunderbird profile folder or file. Has the Anti-Virus quarrantined the Inbox file ?

Do you have any backup of the Thunderbird profile folder ?

If you logon to webmail account, does it have a folder called 'All Mail' - if yes, does it contain the missing emails?

Even in gmail, if a deletion occurs and those emails never went to the 'Trash', it could mean just the 'Inbox' label has been removed, meaning the email is technically 'Archived' into the 'All Mail' folder - also check any 'Archives' folder whilst logged on to webmail account. If you locate emails in those locations you can get them put back into the webmail 'Inbox' again.