AC is now blowing hot air, not cold

The most common reason for the ac compressor to stop working is a low refrigerant situation. There is a switch in the system that will prevent the compressor from working when there isn't a sufficient charge. First thing to check is the pressure in the system. Its very likely there is a leak. If not, the low pressure switch could be faulty or the ac compressor relay but start with checking the refrigerant level.