Your account was probably locked due to trying the wrong password too many times. If you have made 3 incorrect login attempts in 1 minute, your account will be locked for 30 minutes. After 30 minutes, your account will be automatically unlocked and you can try logging again.
Additionally, please ensure that all of the devices such as cell phone or tablet you use to access your account have the correct password as your devices may try to access your account automatically.
Another thing to look at if your account is locked out is any device that you previously had connected to the campus wi-fi.
Apple and Android devices, when connected to wi-fi, will save that password for convenience. This is done so the device can auto-join to that network without having to retype the password over and over on the campus Wi-Fi again. When you change your password on the Account Management System (ams.atu.edu), the system knows you changed your password but your phone doesn't know that you did. It will try continue to sign in with that old password until you clear out your network connections.