When last I parked my Ossa 250 is was running ok. I needed to replace the primary chain as it was very worn. This morning I received the clutch tool to lock it and undo the center nut in the clutch basket. The tool is supposed to "rest" on the crank, but the engine wont turn. Spark plug is removed, the clutch centre turns ok, the outer turns until the chain goes tight, but the crank wont turn.

Please has anyone an idea what may be happening?