Are you using imap accounts or pop3 accounts?

That's because your imap cache has gone corrupt.
Happens sometimes.
The way to fix that is by deleting the cache (while TB is closed down), next time you start TB it'll need a little bit extra time, because the full cache needs to be upd ated, but no online data have been erased.

This is how you clear the cache.
Close TB
Start windows explorer
Go to your mail directory
In your mail directory there is a directory for every account.
The imap accopunts have a subdirectory called IMAP
In that subdirectory you'll see a couple of file pairs with the extensions .TBB and .TBN, delete those
Start TB and everything should be ok afterwards.

If you don't know where your mail directory is located, check it in TB's se ttings:
Options -> Preferences -> System -> Mail Directory