Why won't the air conditioner in my 2004 VW Beetle work?

I have a 2004 VW Beetle 2.0L GLS, the satellite blue edition. I've had it for 7 years and for the past 4 years, the air has not worked. 4 years ago, I had someone try and charge it (just a friend) and he said it wasn't working. The air comes out room temp/ish, but now that it's winter, it's a little different. It blows out like crazy and there is no weird noises or smells happening, just doesn't get cold. What could this be? I am going to change the cabin air filter tomorrow because I read somewhere that it could possibly help. Any ideas?