Its a 15 year old Trane Heat Pump, worked fine until I had the bright idea to install a digital thermostat. Bought the Hunter 44860, turned off the breakers for the HVAC, installed the Hunter, turned the breakers back on and nothing would turn on. Reinstalled the old thermostat and still no AC. I have a service call in with a local HVAC company, but while I'm waiting what can I check?

I know the transformer can get fried in cases like can I check that? I have a nice multimeter and I'm pretty handy with it if need be.

What all can I check while I'm waiting on the professionals to get here? (could be as late as Tuesday...American Home Shield )

Thanks everyone