It could be so many things. Do you hear the air conditioning compressor switch on (with the engine running) when you turn on the air conditioning from inside the car. Get a friend to turn on and off the air conditioning while you listen with the hood open engine running you should hear a clicking sound on and off each time the a/c is cycled on and off or the a/c clutch is not locking up the compressor. This may be due to many things (bad clutch, bad compressor, no power to controls or compressor clutch) but it could be a sign of low charge the system has a low pressure switch which prevents the system from being energized if the system has low refrigerant charge. By protecting itself it prevent catastrophic failure of the compressor and ensuing damage to the rest of the a/c system if the system looses it charge. You will need to have the system checked out by someone experienced in air conditioning work who will attach pressure gauges to your car and see if the refrigerant pressures are OK.