One common problem Thunderbird users experience is an inability to send messages. If you are unable to send messages, try deleting the outgoing mail (SMTP) password for your account. You can do this by going to "Tools" in the file menu, then selecting the "Options" menu. Select "Security Passwords," then "Saved Passwords." Select the URL for the SMTP password (it will have "smtp" in the address) and click "Remove."
Another possible fix is to reset your SMTP password. To do that, you'll have to go to your Internet Service Provider's (ISP's) support website and find a form to reset your email password.
Your ISP's spam settings may also interfere with your ability to send or receive messages in Thunderbird. Check the spam settings, and change the port that Thunderbird uses to send and receive data if you need to.