I had the same thing happen to our 07' Ford Edge recently, but the coolant level was fine. In fact, newer vehicles have the 5 year 100,000 mile stuff, and almost exactly a year ago (and with 52,000 miles) the coolant was flushed from Ford due to a TSB corrosion preventitive maintenance item. Within the last week, the heat just cut off, and the panels were blowing cold air as if the AC were on. I took it in to a independent shop, and they said that inside my radiator hoses was calcium build-up and other junk that was preventing the vehicle from properly warming up. They did a complete flush, drove the vehicle, and slowing down the cabin "cooled" to 90 some degrees when the temp. should be in the 120's with the heat on (the technician had a thermometer with him). I was then told that the thermostat was most likely the cause, so they replaced that. What a difference since heat is now pouring out. The last option could be the water pump since I was told that if the thermostat didn't work, the water pump would be the next thing to replace. Hope this helps.
