Three possibilities here:
1) No fuel. This means bad accumulator, bad fuel pump, bad fuel pump relay. Latter is most likely, so no fuel delivered.
2) No spark. This will have to be investigated on the car, can't help from here
3) Starter not engaging. If you get only a whirr or screech, the overrunning clutch is bad or the solenoild is getting weak and the starter gear isn't engaging the flywheel. If the engine won't rotate, it won't start....!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!