My sons Cheverlot Avalanche sat for a long time while he is in IRAQ, I took it for a drive and now it has no AC, before it was parked it had very good AC, any ideas?

What year vehicle? How many miles? Have you checked to see if the compressor is engaging?

If the vehicle is 6 years or whatever old it’s possible that enough refrigerant charge has been lost and this is preventing the low side pressure switch from closing and activating the compressor.

The system is probably just low on Refrigerant. I’m no expert, but you should be able to hear and even feel the compressor engage when you turn on the a/c. If it is cooling poorly then the system more than likely has a low Freon charge and you should have it recharged by a competant mechanic. Just don’t tell the shop that the truck is low on COOLANT, like my father did. Oops. Good luck.