If your AC is on and there's no cool air in your car then you may have a defective expansion valve.
Regarded by some as the "brain" of the car AC system,

expansion valves

lower the pressure from the refrigerant liquid allowing it to expand and become refrigerant vapour in the evaporator. Your car's blower circulates air across the cold evaporator and into the interior with the refrigerant recycled through the AC system.
Too much of liquid refrigerant entering the evaporator, would mean that the evaporator would freeze, the condensing moisture leading to blocked air flow, which could burn out the blower motor. As a result the liquid refrigerant would remain liquid and potentially enter the compressor and damage the compressor leading to expensive repair bills.