The plug being in/out has nothing to do with turning the motor over and is just a coincidence. I'd eyeball the cylinder and piston through the exhaust port, if all looks good and compression is good I'd have that primary cover off and look for something backed off that jammed the system. If all is well in the primary then pull the top end for a closer inspection and inspect the lower end.
Something went very wrong and will not fix itself. Have you drained your trans oil and checked the plug magnet? This may tell all.
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