depends exactly what happened. could just be a siezed clutch bearing, or it could be the compressor itself. either case, the thing needs to come off the car for a workbench pull-down...
the cheapest way out - find a second hand unit: odds on youll be able to pick up one from here
if youre up to it, removing the old one, and installing the replacement is pretty easy - however, you will need to decompress the entire A/C system, which means once its all reassembled it will need to be regassed.
otherwise, rock up to your mechanic with 2nd hand part in the boot, and say "please install that and regas the sucker".
getting them to supply a new part i suspect will be $$$