Year need to get an OBD II computer plugged into your car. This will interrogate the A/C computer and read off any fault codes - hopefully the unit has already diagnosed the fault itself. Unless you want to buy the computer yourself you would need to get this done by a technician.
One point to check, the temperature went screwy on my A/C. I pulled the control panel out (by removing the Hi-Fi and pushing the unit from behind) and found that the temp sensor which sits inside a hole next the the A/C unit had actually fallen off it's mounting and was lying on the floor inside the dash.