Are you possibly trying to access your account from outside the US ? This is my situation, and I'm experiencing the same thing.
I spoke with an ATT online support specialist who indicated that account access from outside the US was currently broken -- I wasn't able to ascertain whether this ever worked or will ever work (again).
I also haven't tried with my US VPN to see if this is actually the problem, or if she was just blowing smoke.
Regardless, this is a seriously flawed website (with some seriously bone-headed execs who must think that blocking international IPs would provide some sort of "security"). Add ATT to the growing list of unusable websites.