It could be simply due to a loss in pressure of your AC system (needing a recharge and possibly a leak fixed). Or it could be the AC compressor clutch is broken (the problem I had with my 2001 MK4 Golf).
Dealers would say its cheaper/easier to replace the entire compressor, plus you can get them for a decent price at an auto wrecker. Just be sure to bring the car into a mechanic shop to get the AC system evacuated.
I would recommend getting a mechanic to discharge the AC system and do a pressure test to double-check for any leaks before moving forward with a new compressor - could be as simple as no refrigerant.