If you haven't used it for a while there's a chance that you're seals may have cracked allowing the gas to escape. its advised that you use the aircon for at least 15 minutes per month to keep the system lubricated.
Thing is, the pipe work only needs a tiny knock and it can fracture at the joints. It happened on mine a while back. I ended up having a new pipe. It was around £100 and then a re gas.