Check for voltage on the solenoid operating wire (the separate small one on the starter) when the ignition key is in start position. If no voltage, check neutral safety switch and igntion switch.
If you have voltage, the solenoid is bad or the wire connection is corroded.
If you get a click and no starter movement, the starter may be locked up, especially if the headlights go out when you try to start it.
Bad positive battery cable or bad ground from engine to frame will also cause trouble.
And your starter may simply be worn out -- if the solenoid makes a nice click and nothing happens, but the headlights stay bright, the solenoid contactor is shot and you need to replace the solenoid.
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!