Heater not working

It's in the dead of winter in my area, and out of nowhere my heater stops working. Engine temp is normal, and coolant level is normal. Everything sounds the same, and looks the same in the engine bay, no leaks of any kind. No matter what temp I switch to, it just blows really cold air.

Does any air blow out of the vent?

Here are some things to check if you aren't getting any air flow...
1. Vacuum line pinched or leaking.
2. Faulty heat defroster or mode door.
3. Faulty selector switch.
4. Vacuum check valve.