Loss of refrigerant, compressor or compressor clutch inoperative, blown fuses or fusible links. The AC system controls or the engine cooling fan could be inoperative. There are many things to check, if you have a leak, you may be able to see traces of refrigerant oil on the AC components that are bad. If the system is low on refrigerant, you may have to have indicator dye added to help pinpoint the leak. You may have to have a shop equipped with the correct diagnostic tester check it out if it has problems with the electronic temperature control unit(if equipped). Hope this helps.