How can I get my lawnmower restarted?

I have a Hayter Double 3 petrol mower which stopped in use. I cannot restart it as the recoil start cable now only pulls out a few inches. It almost seems stuck but I wonder why it stopped in the first place. Its only the second time I have used this machine. The grass is long, machine set on height 5, running on slow and in gear ? no. 1. Because I have not got the engineer's handbook also incorrectly put in about half a cup of 2 stroke oil instead of 4 stroke to top it up. Any ideas?