There is a shutter that controls the air passage between heated core and Ac core. It must be either stuck at cold position or the motor gear could be sliiping , considering your car has an electric AC control panel. If its manual then bend down from drivers side and try to locate the wire that controls the shutter by moving it from left to right from left hand, you would know the procedure in no time and than manualy assist the latch that controls the shutter, sometimes keeping it at cold for many years can cause it to get stuck,
hope it hepls