Hi my names is Jamie and I will try and help you today.
Are you able to access the account from another computer or device?
Hi, this is most likly relating to BT Yahoo being brought back in house by BT.
BT are moving users to a new portal here.
Please try and login using your full @btinternet.com e-mail address and password.
If this still doesnt work go to this link which should allow you to reset your password online, failing that you should contact BT to do a password reset over the phone.
BT Internet can be contacted on 0800 111 4567, and when prompted say "password reset".