My thermostat is fine - have identical upstairs and down and both turn on heat/ac/fan downstairs, but neither works upstairs, so it's not the thermostat. Crossed the are and G downstairs and fan came on, crossed them upstairs and nothing. So does that mean that the upstairs thermostat isn't getting power? Turned off and then on all circuits in the house to make sure one wasn't secretly tripped. So I'm out of ideas. Any thoughts?

Thanks!

The info you gave is saying you may have lost 24v on the system. Check for a blown low voltage fuse on the control board(simple automotive fuse 3 or 5 amp) or a blown line voltage fuse or tripped breaker inside the unit. Not knowing a make or model of your system it's a stab in the dark.