Yeah it could be a number of things.
Have someone turn on the a/c, while you are listening under the hood for the a/c clutch to engage the compressor. You should hear a click. If you can't hear a click your compressor probably isn't turning. So you will not feel cold air coming from your vents.
You could be, as said before, low on refrigerant or have water in your system. Or you might have a clogged system.
Also, if you do this yourself, remember that it's good to add oil to the sysem. The oil lubricates the pistons in the compressor, so its wise to add oil every time you recharge your a/c system. Our systems need 7oz. PAG (polyalglycol) for every full flush/recharge, but I wouldn't be surprised if there is PAG in the recharge kits you can buy at autozone...