Someone else may chip in and and go straight to a solution, but I can only offer my standard approach to this: isolate the system that has the problem. That means remove one of the timing gears and reassemble the engine. Does it rotate freely then, with the camshaft not rotating? If not, forget the entire camshaft system and concentrate on the crankshaft. If it is the crankshaft, remove the connecting rod and piston and try again. At that point you are only trying to rotate the bare crankshaft. If it won't rotate, it is an endways clearance problem, and there are not many places it can be rubbing against.
Because this happened when the engine was running, it does not seem to be an assembly problem. Something has come apart, on whichever component it is that is the source of the problem. It should be simple once you have identified that component as above. To quote Sherlock Holmes, "when you have eliminated the impossible, whatever remains, however improbable, must be the truth".
