What platform are you on?

You could of course try a clean reinstall

Windows. I really don't want to do a reinstall if there is any way around it because I have 6 years of emails in my folders and the last time I switched computers I lost everything even though I followed directions.

20 minutes ago one singular email came through, though there have been dozens this morning that should have. I sent test emails after that and none of those came through. This is perplexing!

Hi, I have a similar experience since last Sunday 4th July but only with a Gmail account accessed using POP. See my post : https://support.mozilla.org/en-US/questions/1343055 Could you provide more info about the e-mail accounts: provider, protocol (POP or IMAP) (not addresses or any private info of course please!)

Thanks JT

Half the emails are through iPage and the other half are through 1and1 and I have them all set up as POP.

Solutuion - change recurity from SSL/TLS to None, then close Thunderbird. When I restarted it, all my missing email arrived. Remember to turn security back on.