Diagnosis
The control board is getting power but the touchpad is not registering presses. A failed membrane switch panel or a loose ribbon cable connection is the most common cause.
How to Fix It
Unplug the microwave. Remove the outer case. Locate the flat ribbon cable connecting the control panel to the main control board. Disconnect and firmly reseat this cable at both ends. Reassemble and test. If that fails, the entire membrane switch panel needs replacement.
If That Didn't Work
The main control board itself may have failed. Check for any visible burnt components or cracked solder joints on the board. Board replacement is the only fix.
Heads Up
High voltage capacitors inside can hold a lethal charge even when unplugged. If you are not comfortable, do not open the case. Call a technician.