Is the light coming on on the heater panel , if yes when the engine is on idle speed when you switch the air con on can you hear a clicking noise as the compressor starts to work , if not then you need to have the gas checked in the system , also have it checked for leaks , and you need to see if the air con presure switch is working ok ,as this will cause the air con not to work at all , also check to see if the cooling fan is working , hope that helps.
mark
May 2008