Diagnosis
The membrane touchpad or its ribbon cable connection has failed. A spill or physical impact is the usual cause.
How to Fix It
Unplug the microwave. Remove the mounting screws to lower the unit and access the control panel from behind. Check the ribbon cable connection to the main board. Reseat it firmly.
If That Didn't Work
The touchpad assembly itself is defective and must be replaced as a unit. Part number is often on the back of the pad.
Heads Up
The unit is heavy. Get a helper to support it when lowering. Ensure power is disconnected before any disassembly.