You'll track it down. The fact that you got it to start in the interim implies that it's almost certainly a faulty ground somewhere.

I'd check the starter connections first...

Good luck! (Let us know when you find the problem.)

__________________
Eric

1991 560SEL-Euro (214,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)