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.
How to Fix It
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.