The most likely reason is that the system is low on refrigerant because of a leak, but hot air blowing into the interior can result from mechanical or electrical failure in the air-conditioning system.
Modern automobile air conditioners use a type of refrigerant known as R-134, though many people still refer to it by its predecessor’s brand name, Freon. This refrigerant is not something that needs to be replenished periodically — the air conditioner doesn’t “burn” it like gasoline — so if low refrigerant is the reason for the hot air, the probable cause is a leak somewhere.
A technician should be able to find the source of a leak visually or by refilling the system with refrigerant that contains an ultraviolet dye that makes it easier to locate leaks. They can occur in pipes, hoses, the air-conditioning compressor, condenser or the evaporator. In many cars the evaporator is mounted behind the dashboard, and replacing the evaporator requires removing the dashboard, a time-consuming and labor-expensive repair.
Other possible reasons for air conditioners blowing hot air are that the compressor has failed or that an electrical switch that activates the compressor has stopped working. Another reason could be that the accessory belt that turns a pulley on the compressor has broken or is slipping.
Still, all of the above could be operating normally, but a “blend door” in the heating, ventilation and air-conditioning system could be stuck and not allowing cool air into the interior.
Refrigerant refill kits are available for do-it-yourselfers, but the complex nature of air-conditioning systems means that merely adding some refrigerant may not magically transform your hot air into cold.