1. Your engine has frozen, the block has cracked wide open and all the coolant has drained away - terminal engine failure imminent.
2. Your heater radiator has frozen and the ice has burst it wide open and all the coolant has drained away - terminal engine failure imminent.
3. Both 1. & 2. above have occured simultaneoously - terminal engine failure very imminent.
4. The thermostat has stuck open and the engine is taking forever to warm up - terminal failure not imminent, collect a bonus point.
5. The heater valve is stuck, simple to check and rectify - failure not imminent, collect another bonus point.
6. The flap valve that selects and directs the airflow is stuck is stuck or disconnected - engine failure not even on the horizon.
7. It could be none of the above and something else, even more ghastly, has occured - total engine failure happening already, windshield will crack, wheels are about to fall off, the shift lever will come off in your hand, the Rear Sear Retainer Keeper Spring Actuating Pawl Hasp is about to disintegrate and your life will never be the same again.
I would check out 4, 5 and 6, Dom.
Oh, by the way have a Happy Christmas.