Look down at your shifter. Is it in park or neutral? It won't crank if you have it in gear. (Ask me how I konw ) If it is in park, then try starting it in Neutral. If that doesn't work then check the wiring to the starter.

Do you hear a "click" or anything when you turn the key?

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD