Q: 50cc dirt bike 2 stroke problems

I have a mini 50cc 2 stroke dirt Bike and it runs good I ride it around the block every weekend when I got it out this weekend I was pushing it out and the back wheel was stuck and wouldn't move it would only move backwards so I put it on the table and the chain wasn't stuck or anything so I too off the (to my knowledge it's called a single chain transmission) so I took that off what the chain connects to and looked inside there was a l piece broken off but the second chain would still spin so I connected it all back without the piece in and then connected the other chain and it would spin so i started it up and went found the block 2 times fine on the third I did nothing different than the other 2 I slowed down to take the corner then excelersted about 3 meters from the corner the chain came off got stuck around the back tire causing it to stop spinning and I began to skid and i went over and the bike fell on me and there's all white marks on the road