Yesterday my husband tried to install a new TempRite programmable thermostat for our heat pump. Now we have no power to our thermostat but we have power everwhere else. No breakers blow and when we connect the red and green cord the fan does not come on. We have set and reset all of the breakers including the one to the heating and cooling unit. Still there is no power to the lines upstairs. We are unsure if we blew a fuse in the breaker box and want to know how we would know. The voltage detector says we have power. Our house is less then 3 years old so the wires should all be good. Any help would be appreciated because I am freezing cold!

Look for a small automotive fuse on the circuit board inside the furnace .

If its not the fuse hvac 100 mentioned,, could be he toasted the transformer,, you can check with a meter that there's power going in (primary side) and should be 24v coming out the secondary side,, this happens sometimes when you don`t kill the power when replacing a tstat