How long was it between the time you last used the switch when it worked, and the next time you pressed it when it didn't work?

Switches do break (if not the switch itself, then the solder contacts on the logic board) and contacts do oxidize over the years. You need to carefully open your Mac and check the switch on the logic board itself. And I personally would love to see some crisp, zoomed-in photos of your switch when you do that (both top and bottom of motherboard).