1. Check that the keypad is not locked.
  2. Make sure that all keys are responding correctly when they are pressed. In some cases, a key may get stuck and freeze the whole keypad.
  3. Try removing and replacing the covers (if your phone supports that feature). Check that the power and side keys on the cover are properly aligned (e.g. pushed out) to their respective positions to have a proper tactile feel when pressing the keys.