You may find that one of the cookies that BT use has been changed when they upgraded your BTMail. This will probably stop your browser from recognising the logon page and offering the password that you previously stored.
I can not give you specific instructions about how to do this without knowing which browser you are using but you should clear the history and cookies from the browser. You can if you wish do a more targeted deletion by going into your Cookies and Permissions in your browser and just delete the BT ones but you my find it easier just to delete them all.
After you have done that go into your browser's settings and go to the Stored Passwords. Look for the ones that are stored under BT and find your email account one.
Delete it from the stored Passwords and when you log on the next time your browser should offer to store your username and password. Click yes. The next time you log on it should be stored and ready to use.
