Is it a Yahoo email account or a BTYahoo email account?
If it is a Yahoo account you should try clearing the cache and cookies from your browser.
If it is a BTYahoo email account have a read of message 7 of this thread. It may be relevant.
There is nothing that the forum users will be able to do to help so I have notified the moderators of the forum about your problem. Once they have read this they may be able to help.
They are a BT UK based team and if they can help they will reply via this thread asking you to contact them via a link. They are very busy at present so once you have replied to them by the link, it can take 48 hours for them to re-contact you.
It is a bt e mail address and I have had this before. I managed to sort it but can't remember how I did it .
Are you actually able to login? Can you say what the problem is with the BT access?
Strangely although you get taken to a BTinternet login page, after logging in it will still show mail.yahoo.com in the URL