Air conditioner just started blowing hot air

When I came home last night and parked my car, the air conditioner was working great. Extremely cold and no problems. When I go out this afternoon and crank my car to go to the store, I turned on my air conditioner and only have very hot air blowing out of all vents. I checked the temperature gauge and it was normal, on the cool end. I tried defrost, floor, vent, back to air conditioner and max air conditioner. Only hot air blows out. Please help.