1st remove the spark plug for safety reasons. Then turn the mower over with the plug hole in the up position and check the blade and crankshaft for damage.
The blade will then need to be removed and check the crankshaft by looking at the end of it as someone pulls the engine over, with the rope starter, for any wobble in the crankshaft. If the crankshaft is bent you will need a new one or some times you can get that one straightened. I don't like to straighten them because sometimes they will crack and may break off later.
If the crankshaft is OK then you will probably need to replace the blade with a straight, balanced blade. Also check the blade adapter to be sure the key is good and there is no damage to it. Reinstall the blade before trying to start the motor.
If the motor still will not start then you will need to check the flywheel key.