First you need to verify that the engine isn't locked up. Try to rotate it using either the crank pulley bolt (27 mm) or one of the accessory pulleys (PS or AC). If it turns then, you are OK. Make sure you turn it clockwise from the front, normal direction of rotation.
I don't think you have a broken crank (you would hear that when it went), but pull the valve cover and take a look. If the engine rotateks, watch the valves, they should all move the same.
The only way the starter can turn the flywheel without the engine turning is if the flywheel is no longer connected (ie broken flywheel or crank). It is just possible that the flywheel broke free -- it will stay in place on the torque converter instead of flying off and cutting out the floor pan -- and since it is no longer attached, the starter will just spin. No spark since the spark is triggered from the flywheel.
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!