Try turning the engine over by hand, meaning, put a socket on the hex head of the bolt holding the pulley on the crankshaft with the transmission in neutral. If the engine turns over by hand, the problem is more likely starter-related. It is possible if you don't have any grinding or other sounds, other than the "clunk" of the solenoid driving the pinion into the flywheel, that the starter is misaligned and pinion gear teeth don't engage. This will prevent the motor from being energized as the system functions by having the solenoid close the contacts for the starter motor at the end of its stroke. If it is not getting to the end of its stroke for some reason you have to address that. Check the bolts and that the starter is seated against the bell housing flange correctly. Good luck, Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)