Check for spark and for a massive air leak (as in the idle control valve hose fell off or something).
If it ran once, it should run again! SOMETHING has to have changed between when it ran and now!
Also, listen for the fuel pump running while you are cranking. No reason so suspect the fuel pump relay to be good, is there?
Do you smell gas at the tailpipe? If not, suspect a fuel delivery problem. If so, probably ignition.
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!