Who is your email provider? Is this a POP3 or IMAP account? The first thing to try is to restart Thunderbird in Troubleshoot Mode (see Help | Troubleshoot Mode). This will restart Thunderbird with all add-ons temporarily disabled, in case one of them is misbehaving. Not probable in this case, I think. I'm guessing there is a malformed message on the server. To get around that, you can go to your provider's web mail interface and move any messages that arrived around the time you stopped being able to sync to a temporary folder, then re-sync Thunderbird. If any of the messages you moved to the temporary folder are obviously expendable, just delete them. Move the rest back to the inbox one-by-one or in small batches, syncing Thunderbird after each move, until you find the problem one. You may have to delete that one.