+1 to the above. Most likely it is out of gas. they have a low pressure switch that cuts the power to the aircon pump (even when you turn the switch on) to prevent it damaging itself. Most systems, (used regularly as Scott suggests) will hold their gas for about 5 years or so - often more. If the system has a significant leak somewhere, however, you'd be wasting your time trying to get it re-gassed. there are strict regulations that are supposed to prevent discharge of the refrigerant to the atmosphere and big fines from DEFRA for breaches of it, so a proper aircon re-gassing specialist should test the system first and then ONLY re-fill it if it has no leaks. Best go to one of those and get it checked-out. If it DOESN'T have a leak, you can sometimes top them up yourself with cans available from the likes of Halfords or eBay, but you still wouldn't know if there was a leak (until it stopped working again a week later) and the cost of the cans isn't far off what a professional re-gas would cost anyway.