Check controls (temp cable). I'm sorry I can't remember what the control panel looked like inside the dash. Guessing cable slipped off. I don't remember if there was vacuum controls or not (open-close doors inside heater). Since your fan is working it's not likely to be a fuse. Look around in the passenger footwell and see if anything is loose on the heater. When you turn the temp knob there is tension?
If you don't have one, buy a Shop Service manual so that you can 'see' inside without taking things apart.
Ted