I had exactly the same happen to me three weeks ago with my ML500 Merc. I'd started it a dozen times through the day and then it was just flat as a tack. Couln't even raise much of a spark trying to short across the battery terminals with a wire. I charged it overnight, but to no avail. New battery and all was fine.

A few years ago, similar problem but this time it was internal corrosion in the earth strap. The battery was fully charged, I'd cleaned all the terminals incl the ground, but there was so much corrsion inside the earth strap that it wouldn't turn over.

Whatever your problem is, I think you'll find it pretty easy to solve with what the others have suggested.