1997 Ford Windstar engine partially locked up

Engine made no noise. Ran fine with no problems. Park van 4 days later went to start and engine was locked up. Took accessory belt off and starter out and inspected flywheel. All is good. Well today I went out and now the engine is rotating all the way around. Must of been a stuck valve. When the engine was locked up I could not hear any metal to metal contact. Do you think maybe it was a chunk of carbon deposit that broke off from intake system or a stuck valve or? I've always ran Castro 10w30 in winter and 10w40 in summer with about 1/2 quart of Lucas in it. I've never had the upper or lower intake plenum off. I haven't started it yet for the starter is still out of it. I was out marking the crank to give you a better idea of how far the crank would rotate.