Somebody please help

I was driving down the highway, and my car suddenly quit as if the key had been turned off. When I tried to re-start it, all it did was click repeatedly. I bypassed the ignition relay to see if the car would turn over, and it did.... 1/4 rev. Should I start with the battery or the alternator? What does the problem sound like to you? I don't have the money to experiment... and I am ignorant to if the car runs off the battery or the alternator when driving normally.

Thank you!!

Thread already posted in the Mustang forum, no need to post it again here...

Locked.