I agree with PaulC. I had the same experience last weekend with my 87 300E. It ran fine all day, it was deader than a door knob when I tried to start it an hour later. I thought I looked everywhere, but when the road side assistance came out to jump it, the guy told me it was a loose terminal. He tightened it and the car started immediately.
The unfortunate thing is my radio now does not work. Cannot get it to take the security code. I think it got fried when the terminal was loose.
04 Infiniti G35.5 BS