Hi Karen.
If the air con compressor is working when you switch it on then you will probably only need the system re-gassed to top it up to the right pressure. ( if compressor is working you will hear the engine speed change as you switch air con on/off whilst engine is ticking over) If no change in engine speed then poss fuse/compressor fault or no gas at all in system due to a leak( A pressure switch is fitted to system so if all gas has escaped due to a leak it shuts system down to avoide damaging the compressor). A lot of companies apart from main dealers can re-charge the system for a reasonable cost.
Dave
July 2008