First check the pressure.

you can test to see if the compressor works by removing the wiring connector to the pressostat on the accumulator (silver can) next to the firewall in the engine compartment. Put a paperclip in the terminals of the wiring harness, turn on the A/C, and see if the compressor energizes.

If it goes on, remove the paperclip (or whatever) immediately since the compressor shouldn't run without refrigerant because refrigerant carries oil that keeps it lubricated. This tells you that the problem is a low charge (or a defective pressostat, but they rarely fail).

After that, come back and ask around for more information. Chances are, either the system is out of gas, or the control unit on the dashboard is fried. They fail frequently.