As suggested by colin , try the reset button. After that console into to the AP and notice the activities to see whether the AP is elected as the master, unless the IAP is elected as the master you cannot use the username/password to login to the console.
I hope you dont have another IAP on the same layer 2 that is acting as master. You can login only on to the IAP that is elected as master.
Regards,
Sathya