How are you using your email. Is it with an email app or are you using a web browser?
Are you presently a BT Broadband customer or are you paying for your email through a BT Premium email account?
If neither of the above your email account may have been downgraded to a BTMail Basic account which can not be used with an email client or email app.
See link
https://www.bt.com/help/email/bt-email-products
