AC not starting
 

My 1989 N/A 300zx AC isn't working. The belt for the compressor has been taken off from the previous owner. I did a complete teardown of the interior and put in a new head unit. The AC was plugged in properly but only the. illumination of the screens for the AC comes on: no numbers, no heat or cold either. Before I put in the head unit everything worked, but all connections are good to the AC unit after I installed the head unit. It has sat on ramps for 3 months now and I've started it back up just recently, changing the battery and taking it off the ramps. Please help. I need it.