Well, if your power is on to the furnace, then you must check the transformer.
Let's assume your getting 120VAC to the transformer. You must now check to see if your getting 24VAC or VDC out of the transformer. This will tell you if your transformer is shot or your thermostat is malfunctioning. If you don't have a voltmeter to perform this test, you can turn off the service switch, remove and combine the red, green and white wires attached to the thermostat and see if heat comes on. If the heat does come on, then your transformer is good and the thermostat is bad/malfunctioning.