Yep.
Chances are something in the drive train bound up seizing everything to the actual gear on the crank shaft.
It's a tiny little keyway, half moon woodruff style opposed to the more common straight keys.
It's a pain to get back on too as you will soon see, just make sure to use all other parts put back in order and tighten that gear down by keeping the crank shaft from moving..
Something semi-soft through the plug hole to hold the piston just a tad before TDC works.
This internal connection from the engines crank shaft to a big gear reduction to the output shaft is crucial #1.