Try logging into your ATT mail account directly and see if your password works or has mysteriously changed.
To do this, in your favorite browser, go to
www.att.net (not .com), click on the "Mail" button and enter your user name and password. If this works, then the problem resides in the Mail application and/or your setup. If this does not, contact ATT to reset your password.
I am also a little confused when you say you try to "connect to ATT/Comcast DSL." These are two different providers. Did you switch and keep pieces of both?
