Just to test to see if it's the Jiffy problem that Mark found, you could try waiting for about twenty minutes after it locks up to see if it will unlock all by itself (during which time you shouldn't remove power by turning off the car or removing it from the dash or any other way). If it does unlock (belatedly recognizing all of the button presses that you made, IIRC), then you can rest assured that it's definitely not a hardware problem.