DIAGNOSIS The tactile membrane switch behind the 'PROGRAM' button on the control panel has likely failed. These switches get thousands of presses and the conductive pad wears out or the connection degrades.
FIX You need to access the control panel. This usually involves prying off the front plastic panel (carefully, with guitar picks or spudgers) after removing the main housing screws. Behind the panel, you'll see a flexible plastic membrane with printed circuits. The button presses a conductive carbon dot onto this circuit. You can sometimes clean the contact area on the circuit board with isopropyl alcohol and a cotton swab. If the carbon dot on the rubber button is worn, you can repair it with a tiny dab of conductive paint.
IF THAT DIDN'T WORK If cleaning/repairing the membrane doesn't work, the entire control membrane/switch panel assembly needs to be replaced. It's a single part that connects via a ribbon cable to the main board.
HEADS UP Be extremely gentle with the ribbon cable connector when you disconnect it. The locking tabs are fragile. If you break that connector, you're now replacing the main board too.