Have a Honeywell simple digital non-programmable thermostat. The AC is fine. When I switch from AC to "off" or "heat" position, the display goes blank. The heat does not come on, but the blower will if I switch it from "auto" to "on" obviously blowing cold air. This is an all in one (gas heat and AC) large unit outside the house. Over the summer, a repair man came out and did a whole cleaning and whatnot on that unit for the AC, we have never had trouble with the heat. I wasn't here when he was so I don't know if he tested the heat when he was finished (it was summer) so I'm a little suspicious that maybe he accidentally disconnected something, flipped a switch, etc. That would cause the heat not to work.

I discovered the green, yellow, white and red wires inside the outside unit which were obviously connected to the ones running to the thermostat inside. A couple of them were disconnected where they were supposed to be twisted together with those little plastic things. That bundle of wires runs through the front of the outside unit to the ground and I know I've hit that with the weed eater, it must have pulled them apart on the inside. I reconnected them and now have no problems.