Please give us more specific details. What happens when you try? Any and all error messages, in full and precisely quoted, would be most helpful. Also, check with your email service to confirm that your server settings are correct.

By the way, Thunderbird is the program that runs on your local computer, which interacts with your mail account(s) out there on the Internet. You don't actually log into Thunderbird itself, you are logging into your email service.