Is you email working when you use a web browser (webmail) . Try using the "Email" butto at the top of this page. If you can log on is there is obviously nothing wrong with you account so do not change the password as that can cause more problems.
Are you presently a BT Broadband customer or are you paying for your email through a BT Premium email account or did you migrate to EE Broadband?
If none 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 apps.
See link
https://www.bt.com/help/email/bt-email-products
If you are one of the above I would suggest that you do nothing. It is most likely a BT server problem which BT will be aware of and the email service will be restored as soon as possible.
