engine shut off while driving now only clicks once upon cranking

My friend was driving her car yesterday after we changed out the thermostat housing. She was going up a hill heard a noise that she described as like a fan turned on. Lost all her power and then the car quit running. When we try to crank the engine over it will do a single click and that is it. Battery power is good as we even hooked up to a jump box just to make sure. I am at a little bit of a loss on where to start.