1993 Mercury Tracer air conditioning stopped working

1993 Mercury Tracer Mileage: 142,000. Recently I had the head gasket replace and valves redone. The guys at the shop took about four weeks to do the repairs. I've had my car about two weeks now and the air conditioning compressor is started making a loud noise. The air conditioning was working then stopped when the noise started. I looked at the compressor and I think (hope) it may be the clutch. It has a little grease leaking in the front on the clutch. The guy at the shop is saying the grease is from the compressor. I really don't know. In addition, if I replace the clutch will I need to decompress the system or is the clutch a separate piece? Also, Is there any chance the guys at the shop could have done something to cause this? Thank you for your site.