My lawnmower won't start or turn over?

I neglected to cut the gr for a long time so it was really long. So I knew going in I would have to back up every few seconds because it couldn't take it. It stalled and I started it again it was good for another minute or two then it gave out for good. It won't let the crank pull back so I cannot start the mower can anyone tell me whats wrong. I am about to open it up and try to figure things out so any ideas or information would help a lot thanks.