2001 Chevrolet Prizm?

I was looking at a 2001 Chevrolet Prizm. It's an automatic transmission, and has 118,602 miles on it. They said that when they were driving the car last fall, it suddenly coasted to the side of the road and quit as if someone turned the key off. They said the engine turned over fine last fall, but the battery is dead now. (They kind of confused me a little...I'm basically copying what they wrote in their description.) Does anyone have an idea what is wrong with it? The cost to repair it? Thanks.