DIAGNOSIS This is usually a failure of the membrane touchpad or its connection to the main control board. Spills or grease seepage over time can damage the pad.
FIX Unplug the unit. The touchpad is a separate assembly. You can try reseating its ribbon cable connector. This requires removing the outer casing, which I'd leave to a qualified person due to high-voltage components inside. If you're not comfortable with that, call for service.
IF THAT DIDN'T WORK If the connections are good, the touchpad itself is faulty and needs replacement. It's a single part that includes all the buttons.
HEADS UP Don't try to 'jump-start' it by pressing buttons repeatedly. If the pad is shorted, you could cause further damage. The safe move is to unplug it and get it looked at.