Couple of things...First run a 12V pos lead directly from the battery to the plug @ the compressor to verify that the clutch is good to go. You should hear it engage. Plug it back in. Once you've done that, got to the pressure switch located near the condenser on the passengers side. Unplug it, then bypass it by inserting a paperclip into the end of the plug. If your compressor engages now, that means one of two things bad switch or lack of pressure in the system. My bet is a leak let out the pressure and now the switch is doing its job and not letting compressor turn on. If Both these tests check out, then check the temp switch near the thermostat, this will turn off the AC when the engine temp gets too hot. It could possibly go bad too.