Heater doesn't work. I have the round digital honeywell thermostat. Usually I switch the little

Question

I have the round digital honeywell thermostat. Usually I switch the little k**b to "heat" and the other one to "fan" and press the up button to increase to desired heat, the hot air comes out. Yesterday, I did the same thing and it isn't working anymore. I just feel the cold air coming out of the vent. I've been messing around the set button but still nothing. Please help.