When your cars air conditioning starts to blow hot air our suddenly then this is an indication you have a refrigerant leak, AC electrical problem or clogged condenser.
Your car uses a refrigerant in the AC system to remove heat out of the inside of the car making the car become cooler. If your AC system develops a refrigerant leak in the system from a leaky connection or damaged AC hose, the refrigerant will evaporate and make the air conditioner loose its performance. If the AC system has an electrical problem such as a switch or wire that has shorted and blown a fuse, then the air conditioning system will malfunction.
If your air conditioning system starts to blow hot air out over time, this is a sign that there is a small leak with the system or you could have a clogged condenser. A small leak in your system will slowly drain the refrigerant and the air coming out will gradually get warmer.
The same happens when the condenser is clogged. The condenser in your car acts as a mini radiator that will remove the heat that the refrigerant absorbed from in the car. Over time the condenser can get clogged from debris and dirt coming from the road. This can slowly reduce the performance of the AC and your car will start to blow warm air. In order to fix this problem you need to remove the dirt that has built up on the condenser. By using a little liquid dish soap in warm water and spraying it on the condenser will remove the dirt. The best option is to put it in a garden sprayer and apply it. Let it soak for about 10 minutes then rinse off with clean water. It is important to remove all the soapy water so it does not leave a film on the condenser.
If you are unable to determine the problem it is best to take it to a mechanic that specializes in automotive air conditioning repair.