Ok, your coolant level has nothing to do with your A/C. It's a completely seperate system and work independantly of the cooling system. Like others have suggested, the first step would be to hook some manifold gauge's up to the low side port(big line) and see what kind of pressure you're reading. If it's below 35psi, add a can or two of Freon(12oz's). If it's above 35psi, then you've got a more serious problem. Could be an a/c compressor going out(usually the case) or a blockage in the system.

Let me know how it goes.

~Johnny