Try loosening the screws that hold the pull start on and try to wiggle it to see if it will retract if not take it off very carefully as it may explode.

If it made a snap when you pulled the string the spring is broke.

Those pull starts are hard to put back together if it is just a jam and comes undone then great but if it is more than that just get a new one it will never be the same after you mess with them.