Central heat and AC unit worked fine this morning; cycled on and off when temperature dropped in home. Now it's getting much colder and unit won't come on. Tried setting thermostat on "blower" and it came on; blew cold air only. Set it back on Automatic and Heater and nothing! Please help!

Try turning power off to the furnace wait a minute then turn it back to on. What happens? What brand? How old? What efficiency?
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