Should you ever find that your thermostat is not getting enough power, the first thing that you’ll want to do is check the circuit breaker. Once you’ve found the breaker that controls the furnace and air conditioner and see that it’s tripped, turn it back on and see if that helps. You can also check the power switch on the side of the HVAC unit to make sure it’s in the ON position. Sometimes the most obvious place is the best place to start.
You might also need some fresh batteries in your thermostat to get it working. Even if the batteries still have some juice in them, there might not be enough for them to adequately power the thermostat. If neither of those options works, you might have to use a multitester to see if your unit is getting enough volts.