2004 pontiac sunfire shut off when i was driving on the highway and wont

start anymore? ok i was driving on the highway yesterday and my car just shut off it was just the engine the radio still played. anyways i took it to autozone to check the battery and they said it was fine. so anyone have any ideas on what it can be? i was told it can be the alternator went out. but before i go out and buy a new one i wanted to know if anyone had any ideas before i pay alot for a mechanic. just anything so i have some idea to start at