First:
If the plugs are out, engine can't be hydrostatically locked, but if a valve broke a stem and fell into a cylinder...I'm just saying... shine a light into the cylinders (bore light if you have one), or pull the valve cover and make sure all valves are intact. Both jobs way eaiser than pulling the head.
Second:
Anything that rotates with the crankshaft could break and lock the engine, ranging from distributor, oil pump, or cam chain.
I suguest that you (1) pull the dizzy and be sure the shaft turns. (2) Turn the oil pump, although if it is the culprit, removing the dizzy and drive gear from the cam shaft takes it out of the loop and if the engine will now turn over, the problem is a locked up oil pump. (3) Feel around the bottom of the cam chain cover for any dimple(s) that might indicate a broken chain jambed between the crank sprocket and the cover. A loose nut or bolt from the cam sprocket or chain tensioner could also be jammed in there too. Turning the engine backwards might free up the jamb enough to indicate that the problem lies in this area.
If you have no joy after these checks, pull the pan and look for a broken crankshaft or connecting rod.
Hope you find the problem quickly. Good Luck!