Why my 99 Camry Engine Stop While I Was Driving.?

I was driving on the freeway here in NJ and traffic had slowed down to about thirty. When it picked up I tried to accelerate to the free way speed...seemed like the engine chugged a bit then no acceleration saw engine light come on...I turned off the AC tride to give gas again but nothing. Wound up coasting it to the shoulder. Try restart couple of times but wouldn't kick in and hearing hi pitch sound like sound winding up fast. When AAA came out I tried again. Tow guy said that the alternator turned the belt and wasn't sure what it was. Will be having it towed to mechanic but want to know what to expect.