If you have not used the email account in the last 150 days it will have been deleted.
See link
https://www.bt.com/help/email/why-has-my-bt-email-address-been-suspended-or-deleted-
If the above does not apply you will need to contact BT as suggested by the moderator in message 6 to see if they can resolve the problem.
I use it all the time, just not through the webserver
@paul_round wrote:
I use it all the time, just not through the webserver
That was not clear when you said "I have used the webmail before, but not for a while, so have forgotten the password" so I thought it best to check.
Given the information that you have supplied it would appear to me that your email account has become "detached" from your BT Broadband account and as such there will be no way that any of the forum members will be able to assist with this.
I think that the best course will be to re-contact BT Customer Services 0330.1234.150 and ask to be put through to the BTMail Team as I doubt that the first line agents will have the ability and knowledge how to resolve this for you.