Is it doing anything when you try to start, make any noise, clicks etc? Do any of the other electricals cease to function.
If no noise of any type, and the other electricals (lights, radio, buzzers) work, it could be the shifter bushings, neutral start switch or even corroded connections. spjohns1 recently posted that he was having difficulty on his 300D, and cleaning the connections at the relay or junction block cured the problem. I don't remember if it was in Tech Help or the Diesel forum.
Check out his post
83 300D Turbo
Hope this helps.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.