Sometime when I apply pressure to the pedals while riding, the chain jumps from the plates, One day it got stuck when I was pushing on one foot, I went over the bar and fell pretty bad.
What could it be? I changed the chain because it was stretched and somebody told me that it could be the cause. The problem persist, I even stopped using the middle plate which is the one I always use.
Search for chain/chainring wear. If the entire drivetrain is so worn out, a new chain won't help. You'd have to also replace the chainring and cassette.