Will it start providing heat again if you bump up the temperature wheel significantly?
If so, then you probably have a decomposed tube behind the globe box that goes to the temperature sensor in the top middle of the dash. That tube was made of foam and just falls away after years of age. The tube's purpose is to allow a small amount of air to be drawn through the temperature sensor, to sense the cabin's temperature level.
When the tube drops away, then what you sense is the temperature inside the dash, where all the heating ducts are located - much warmer than the cabin. So the ACC thinks you are nice and toasty and throws in cold air.
So, if you get heat by turning up the temp wheel all the way, let me know and I'll give you more details on the foam tube replacement. (Basically, if you remove the glove box you should see where it goes.... and perhaps the remains of a foam tube.)