AC hisses and blows cool air

Hi all,

It's about summer here, and I noticed my 1990 190E 2.6's AC does not blow cold air, just cool air from the vents. It also makes a slight hissing noise when the AC is activated. My car had been sitting for a few weeks before, and after redriving it that's when I noticed the AC not working the way it should. Do I need freon (sp.) or some kind of AC repair? Is having a non-operating AC a serious problem that may affect the function of the car? The car drives fine, but I just need to know if this problem will lead to more serious mechanical difficulties. Thanks in advance.