10 tooth sprocket

hey guys, i was rideing my my one night and it keep reving high and not wanting to catch any gears.Eventually a gear grabbed once i stopped at a light my motor turned off and didnt wanna start.i tried to ride it and release the clutch but there was no ignite.i got home took off the cover the sprocket was loose and it looked like the chain was eating at the cover.does any one have pic of how to istall or look for this woodruff key?please help out ive had my motor for four days and i already miss rideing it.

thanks for ur guys help i replaced the woodruff key.i couldnt tell where it was because it was sheared.moreno valley is in california but people here call it moval.