What can cause a car to completely shut down while driving?

just bought a used Chevy Equinox with lots of miles. My daughter was on her way to school and it completely went dead while driving. I went and turned the key and nothing at all, dead. Next day before tow truck arrived, I tried again and there were dash lights and clicking sound. He gave me a jump and it fired up. Few days later, same thing, only she was not driving, but it would not start, dead again.