My thermostat went blank and my heater is not coming on. I replaced the batteries and it is still blank. What do I do

First make sure you have 120 Volts going into the furnace.
If not check to see if the blower door has a switch on it that kills the power to the furnace when the door is removed. Test door switch to make sure it is good.

Then see if you have 24 volts coming out of the low voltage transformer.

After these tests report back to this thread with results.