Answer:
It depends on the engine type. If it is the older type with a starter clutch, the ball bearings could be froze up in the starter clutch. Just take the flywheel cover off (the thing with the rope on it) and pope the cover off of the starter clutch. Clean the ball bearings and the surfaces and oil it with motor oil and reassemble. The starter rope could be tangled itself. You can remove the flywheel cover and the rope should pull out easily and retract back in easily. If it is still stuck, its tangled in the flywheel cover or the spring that retracts the rope back in is damaged. The newer style engines with the starter clutch that has the arms fly out (the start clutch is part of the flywheel cover instead of part of the engine like on the older style). These ones are harder to prepare and usually it is better to replace them. If you have a salvage yard nearby you can usually find a junked engine you can pull one off of. These small engines are very simple and fun to work on. It could be a great learning experience and I recommend you try to fix it. Many people through good mowers out each year because they don't bother trying to fix simple problems.