2003 Nissan Sentra AC doesn't work at all

I took my car to get the AC fixed and all they did was put more 'coolant' or something in whatever needed it. At first, after they did that, there was AC...and I was ecstatic, but then after 2 days it went back to not working anymore. I'd like to repair it myself so I don't end up wasting more and more money. I don't know much about cars, so I'm going to buy a Haynes Manual for my car today to try and address the problem. Do you have any suggestions? I would be extremely grateful for any help you could provide. Thank you,
Joe