This usually happens at the most inopportune time. You go out mow the lawn, pull the rope and it comes out in your hand. Here's how to fix it yourself.
First, we'll talk about a simple rope replacement. In most cases you will have to remove the blower housing from the engine to get to the underside of the rewind starter where the rope is. We'll assume at this point that the spring is ok and you can just wind it back up again. You can check the spring by turning on the hub in a counter-clockwise direction and see if the spring gains tension. Do not try to remove the spring from its holder or you may end up having to take the mower in for service.
1. Remove the rope from around the hub. Since there is no tension, it should come right out. If you are lucky and it broke near one end, you can probably just re-use it. If it is badly frayed or too short, you will need to find a replacement at a local hardware store.
2. Attach the handle or tie a knot in one end of the rope so it will not wind all the way into the hub. You also may want to burn the rope ends to keep them from fraying and causing a problem in the future. Set the rope aside where you can grab it once you wind up the spring.
3. Wind the hub in a counter-clockwise direction using your thumb to keep it from unwinding, until the spring is tight. Then let it come back slightly so the rope hole in the starter housing lines up with the rope hole in the hub.
4. Stick the rope through both holes while holding the hub from unwinding. This is tricky because you don't want the hub to unwind on you before you have the rope secured in it. A pair of needle nose pliers or a small screwdriver or pick may help get the rope through.
5. Tie a half hitch knot in the rope after you have it through both holes. While holding the rope, release your hold on the hub and let the rope slowly pull itself back into the starter.
6. Put the blower housing back on and your ready to start the engine & mow.