Since the starter rope will not pull the rope back it, it is likely that the starter will need to be replaced to fix this problem. Using the model number of your tiller and possibly the engine model number as well, you may be able to order a replacement starter and replace it yourself. The starter on many engines is riveted into place. You may need to drill out the rivets to remove the existing starter assembly. The replacement starter assembly will normally have a set of nuts and bolts to secure the new part on the tiller engine.
If you do not feel confident repairing this problem yourself, then you can have it repaired by a Sears technician. Here is a link that will provide the nearest drop-off locations:
Sears Home Services