what causes the heat to go out in a 2008 ford mustang. At first it was blowing warm to hot air now it just went out completely

Check to see if vacuum operator behind the dash control knobs is still working. When the car is hot, set the air flow direction to "dash/windshield", then turn the temperature to full cold (listen for the A'C compressor to turn on and the temperature to get cold coming from the dash vent). Once you have felt the cold, turn the temperature knob slowly to full heat and listen for the A/C pump to shut off, while listening for the vent "doors" in the dash board to open and close as they go from cold to hot orientation. You can also see if under hood vacuum valve for switching from cold to hot while operating correctly as well (look for the heater hose coming from the water pump headed towards the firewall). If everything operates and still no heat , then you need to change the vacuum operated temperature controller, located behind the dash board (it is connected to the knobs which control A/C-Heat, Air Direction (floor, dash, windshield) and Fan Speed).