Are you a BT Broadband customer or are you paying for your email account through a BT Premium email account?
If neither of the above you will have been "downgraded" to a BTMail Basic account which can not be accessed using an email client such as Thunderbird.
See kink
BT Email products | Types of BT Emails | BT Help
If you are one of the above can you answer the following questions:-
Has this just started, have you previously been able to use Thunderbird?
What happens when you try to access your emails using Thunderbird. Do you get any error messages?
Do you have T'bird set up as a POP3 or IMAP account?
Have you checked the settings as per the following link
BT Email server settings | BT Help
