Are you receiving error messages?

No error message. When "Get Messages" is pressed I'm told there are no messages to download, which isn't true. I can go to my email client and get messages via their web mail link.

Thunderbird was downloading messages on the Dec. 22 and then on Dec. 23 wouldn't. No settings were changed.

Is this a POP account? All I can guess is that the popstate.dat file is corrupted and thinks it has downloaded the current messages. That file in the profile could be deleted, but then Thunderbird would download all messages. If there are only a few, that would be okay, but some people keep thousands of old messages on server. If you want to try this the file is in the Mail\<accountname> folder in profile. You can easily get to the folder by clicking help>troubleshootinginformation, scroll down to 'profile folder' and click 'open folder' and then exit Thunderbird. But my idea is just a guess. If you have thousands of messages on the server, this may not be a desirable move.