Why did my wall-mounted garage door opener button stop working?
I'd start by opening up the opener, and making sure the wires for the switch are securely connected. 

Inspecting the cable for damage, would be the next step. This would include testing the wires for continuity, to make sure there's not damage that couldn't be seen.

Check the control board in the opener for any fuses, and make sure any fuses are good.