Sounds like your compessor locked up. With the AC off the belt pulley free wheels on the compressor, when you turn it on an electric clutch engages the compressor. Even it there was a short in the system somewhere (this would blow a fuse) your engine would not die. This is why I believe the compressor is locked. You can try to turn the front of the compressor by hand (*Engine off, keys in pocket) and see if it is locked. The part you want to turn is on the front of the pulley, it has 3 rubber bushings on it. Good luck
