If your air conditioning in not working, or working but not up to par, there can be a few different things that can cause this. There can be anything from restrictions in the system to your blend air door vents not working properly. First and foremost is look to see if your compressor is coming on. If all of your freon has leaked out or even if the freon level is a bit low, the computer will not turn on the compressor. This is to prevent damage to the compressor and the components. This is very common! If the compressor is cycling on and off, than the system is just slightly low on freon. This will cause the AC to work, but poorly. Look for moisture around the outside of the AC fittings. This will indicate a leak. If no moisture is detected, you might just be able to get away with recharge.
I think it turns off.
It it turns off, then chances are you have an electrical issue. You could have a bad AC pressure switch or even a bad evap temp sensor.
If the compressor continues to run, then chances are the compressor is bad or you have a restriction in your AC system causing this problem.