I have an ATT Mail account (had it for years) and never had a problem logging in until today. On my primary PC when I log in with my User ID and Password I get the message stating: "The User ID and Password combination you entered does not match our records." Interestingly, I could still access the email account from another computer by using the same old User ID and Password combination. I can also access my main ATT account from the first computer using the same User ID and Password combination; just can't access the email account.
I remember reading a while back that this can occur when someone has hacked your email account, and that you should change your password, but do it on another computer, otherwise the hacker will see you new password and you still won't be able to access your email account. Don't know whether or not that's true, but I changed my password using another computer and still have the same problem on the first computer not recognizing the User ID and Password combination.
P.S.: In case it means anything, the following is the URL link that shows for my email account login screen: