DIAGNOSIS This usually points to a failed control panel or a loose connection to it. If the fan and light (which are often on separate circuits) work but the touchpad is completely dead, the main user interface board is probably the issue.
FIX 1) Unplug the microwave. 2) Remove the screws holding the control panel cover in place—they're usually along the bottom front edge. 3) Gently pull the panel forward. You'll see a ribbon cable or a wire harness connecting it to the main board. 4. Carefully disconnect and then firmly reconnect this cable. Plug the unit back in and test. If it's still dead, the control panel itself likely needs replacement.
IF THAT DIDN'T WORK There's a slim chance the main control board is faulty and not sending power to the touchpad. Before replacing that expensive part, double-check that the door switches are engaging correctly. If the main board doesn't get a 'door closed' signal, it will lock out all cooking functions.
HEADS UP When handling the ribbon cable, be gentle. Don't bend the connectors, and make sure it's seated perfectly straight when you plug it back in. A slightly crooked connection is a common cause of intermittent problems.