What are the causes that prevent a 1999 ML320 from starting?

1. Dead battery 2. CPS is bad and needs to be replaced 3. Bad starter motor

Hard to tell unless you can give more details, does it make a sound when you try and turn it over? A classic dead battery sound on Mercedes is a click noise when you try and start it, sometimes it confuses people and they think the starter motor is bad but that's not the case.

I had a similar problem and it turned out that the grounding strap on the starter motor just disintegrated.