I think she might be dead.....

[clonak]

Seen it before, Definately something hit the spark plug, Possibly the piston, or as Alan said, something dropped into the cylander.
As to why it wont turn over, probably bent the piston/conrod so its jammed up. Or the conrod has snapped. Either way, head and cylander has to come off, Get the tools out and more pics.

[OldNTired]

Originally Posted by alantf

I'm no mechanic, but do you think it's possible that a valve has sheared off, dropped on top of the piston, then the plug's struck it.

I don't think we'll ever know, until the head comes off.

Going by the pic, and his story of what happened, I'd say you are probably correct.
That central stem on the plug is pushed over, and the deformation around the edge
indicates that it was definitely taking a beating; the only thing that could
have done that is a valve.

[Water Warrior 2]

Looking at the pics and reading the comments I would say you are in for some exploratory wrenching. As suggested, you may as well pull off the head to investigate the condition of things. Wishful thinking and guessing will get you no where.
The riding season is over for many of us and we need some one to help and coach for a while.