I'm pretty sure its the serpentine belt and possibly even the water pump. The belt has come off, but it isn't broken. The car started losing power. First, the battery light came on and the lights dimmed. Then the airbag light came on, the gauges went crazy, and the lights dimmed even more. The car then started running rough before shutting off. Now, the car won't start.
It sounds like you may have a broken serpentine belt or a idler pulley that may have caused the belt to fall off. I would suggest that you get a mechanic, such as one from YourMechanic, to check all the idler pulleys and verify they are good and if not replace them with the serpentine belt. The water pump is turned by the timing belt. This belt should be replaced every 100,000 miles and you should also look to replace your water pump, too. If the water pump is bad normally they will start leaking water. You will see water dripping in front of the motor on the passenger side when it starts to fail.