It could be the blend door lever or temperature control cable stuck or broken behind the dash. Flip down the glove box door and you will see the end of the cable going to the lever on the A/C box/case. Try and move the temperature control knob on the dash while looking behind the glove box door and see if it moves the lever back and forth. Would be good if you have a way to check pressures but from what you describe I would check the cable and lever first.