Often it can be because the seals have gone if it hasn't been used. You're
supposed to use them for at least 15 minutes a month (if not a week). A
cheap and cheerful way is to get a top up cannister from Halfords. For £17
or so it will repressurise the system and if it springs back to life then
you'll know the problem. Replacing seals is probably a specialist job
because of the gasses involved. A simple refill will be about £50. It's
possible that there's no power to the electric clutch in the compressor due
to another fault but the clutch won't engage if there's no pressure.

If you're anywhere near Poole I'd be happy to take a look

www.poolecool.co.uk

Lack of use has caused your seals to dry up, and all the gas to escape. You need
to use your aircon all the time, get in the habit of running it on at least a
weekly basis.
Note that it doesn't have to blow cold air - it works as a dehumidifier whatever
you set the temperature to.

Get out your Yellow Pages, and phone some local AirCon guys. You'll be able to
get it regassed for around £50, and this will include them adding a dye to the
system. So if it does have any leaks (unlikely - it's prolly just the seals as
above), they can be spotted.

As Malc says, try one of the regassing bottles from Halfords, they also
contain a sealant which will help if the seals are a bit dry. If it then
still won't work, it's a specialist job.

As others have pointed out, if you do not use your a/c during the
winter, the seals glue up and the first time you turn on you lose all
your gas.

So why would anyone use a/c in the winter?

Simple. There is no method quicker than turning on the a/c to de-fog
inside the windows. As you will know from seeing a pool of water under
your car below the a/c, the a/c reduces the humidity of the air and,
especially if you also run the heater, your windows clear in a flash.

And if you do this two/three times during the winter you will probably
save your seals and your gas.

I thought about doing it myself but decided to let a specialist do it, It
seems they have to vacuum the system  before re-gassing it. If you know how
to do it try the a/c kits you can buy but I thought £60 for someone to
re-gas my car was not bad . I know I would have botched it up.