I replaced my furnace and thermostat about 4 years ago. But last weekn, when I tried to turn on my AC by switching the thermostat to "Cool" position, the AC unit was not on. When I further switched the fan to "ON" position, the fan didn't work either. Does anyone have clue about what went wrong?

Thank you in advance for your help!

Could be a breaker needs to be reset or a fuse some /most furnaces have a fuse in the control board
Look for loose wires small tstat wire/low voltage or a high voltage wire just be careful when looking around in a furnace