No flashing or blinking lights or numbers. No read out at all on thermostat.

No batteries, it bleeds power off the furnace I believe. York natural gas furnace. I checked the breakers, they are all on. The pilot light is on.

This has happened before, about 2 weeks ago. I had a service guy come out, he tested stuff for hours, and found nothing definitive. In the process of tinkering, the thermostat came back on but the service tech could not tell us why. We left it at that. Now it is out again.

I know there is switch that shuts off the furnace to prevent overheating. Before I call for service again, I would like to try to reset that but I have no idea what it looks like or where it is located within/around the furnace.

It is a York. Model number P4UG012N06501A

Any info would be appreciated.