Diagnosis
The membrane keypad has failed or the ribbon cable connecting it to the control board is loose. Spilled liquids or physical wear are common causes.
How to Fix It
Unplug the microwave. Remove the control panel cover. Locate the flat, gray ribbon cable connecting the keypad to the main board. Disconnect and firmly reseat this connector. Reassemble and test.
If That Didn't Work
The membrane keypad itself is faulty. Individual button contacts degrade. The entire keypad assembly must be replaced as a single unit.
Heads Up
Do not use liquid cleaners directly on the keypad. Moisture seeps behind the membrane and causes internal short circuits.