The other day my friend turned off the A/C and it just stopped working since then. It pushes warm air since. It's so hot outside that I don't know if it's air from outside, the engine or the heater. When I turn on and off the A/C I can hear i kind of whistle when I turn it off.
Can anyone help me?

 

You might need to charge your system. Pick up one those R-134a recharge kits from the parts store or walmart or whatever.

 

Have you checked fuse 25 (10amp) in the PDB?