Air Conditioning Stopped Cold

1990 300 SE: My A/C won't blow cold air. Last week while driving, I heard some unusual pneumatic air noise (soft hissing) for about a 10 minutes, then it stopped. Didn't think much of it until we had a warm spell here in Atlanta--then, you guessed it. Turned on the A/C, and got wind but no coldness.

Anything I can check before taking it in to the shop? Any ideas on what could be happening? Was that my refrigerant I heard leaking out?

Thanks