Heater went out, but…

Our HVAC heat is gas. It’s very cold, in teens here. I woke up at 2am cold and when checking thermostat, I saw it was set at 68 but the house was 60 and dropping. The air coming from vents was cool. We moved to living room and turned on unvented gas logs for heat. Around 6 I noticed it was very warm in room (72). I checked vents and warm air was now coming out. Thermostat set on 70, house temp 70 (hotter in LR due to gas logs). So…..What could cause gas heat to stop heating for 4 hrs then start back working?