I just had the same thing happen to me with a MagIII. For the second time. It was about 15 dollars to have the local lawnmower/small engine repair shop replace the spring. I found that the spring itself was broken, and that rewinding it myself still didn't work...plus, what a headache!
Now I just have to deal with the thing not turning over. For ten years, it hasn't taken more than two pulls to start. In fact, the day the spring broke, I was able to wind up about 6" of cord, and that was enough to start it again. Since the spring was replaced, they've put new coils in and it still won't start; they say that the cylinder is scored and might not hold compression. Funny how it only happened after I took it in, though. Oh well, when it hits the fan...
Good luck. you should have an easy fix.