What you will need to do is dependent on wether you can work on the car yourself, or if you would rather pay someone to do the work for you.

If you don't want to work on the car yourself, take it to a good shop, and have them fix it.

If you can do the work yourself, go to an auto parts store, and spend a couple dollars on a can of the R112 refill, that comes with a pressure gauge on the can, and has leak detector dye in it.

Trace down the a/c lines, and hook up the refill canister to the proper line, and follow the instructions. If the lines had no pressure according to the gauge, you might have a damaged line, or a leaking valve.

If the lines did have pressure, then you might have a bad pressure sensor on the system.
If you can find the sensor ( I couldn't, but its 15 degrees in my garage right now), you can disconnect it, put a jumper wire in the connector, and test to see if you then have a/c.

After that, its then the a/c unit that is bad.

BC.