Make sure the key and the a/c switch are on with the engine off. There's a single wire that goes to the a/c clutch, flow that wire to the connector and check for power going to the a/c clutch. If there's no power then your either low on freon or the lower pressure switch is bad. More likely the freon leaked out. Oil residue on any a/c component is a sign of a leak. Flow the a/c line and look for oil residue, also check the underside of the a/c compressor just behind the clutch for oil residue. Look at the a/c condenser behind the front grill and make sure a rock or something didn't put a hole in it.