I have a similar problem, the only difference is I don't hear any sounds when pressing buttons. Holding the power button for an extended time does not help I hold (simultaneously) the lap, power, and enter buttons for about 10 seconds. It's happening far too often for my liking.
I have a similar problem and the FAQ provided by Garmin does not list this under trouble shooting despite that it has been a problem for years.
Ugh! It just happened to me for the first time. Same thing here...its frozen, the screen is on and I didn't hear any noises when pressing buttons. Thanks for posting this...it worked.