I have a 1975 MB 240D, I was driving to work one day and I starting losing power, like I was running out of fuel, then it just died. I tried to start it and all I heard is "Clunk", so I replaced the starter and the solenoid and both battery cables to make sure I had a good connection, still nothing, I then replaced the ignition switch and have installed a brand new battery. I tried to turn the motor over and can't budge it, but when it stopped running, I didn't hear anything break. Any ideas?

Sounds like your engine may have seized. Were there any indications of oil pressure problems before it stopped? Try putting a 27mm(?) socket on the bolt on the crank pulley and turn it over by hand. It should be relatively easy to turn over by hand. You could also try turning it over by using the large nut on the power steering pump. Report the results, and we can go from there.
You'll also want to post the question on the diesel forum. Lot's of people there to help sort out the problem.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13