if you hit something then more than likely you bent the shaft that the blade is on. this will keep the motor from turning when you pull the rope to start it .the best thing to do is just buy a new mower

Did you change the blade? If not the blade could be jammed against the mower. You need to check the blade for damage and check the crankshaft to see if it is bent. Remove the spark plug wire before messing under the mower. The shear key is between the flywheel and the crankshaft. This would not prevent the engine from turning, only starting. Remove the blade and see if the engine turns. Unless you broke the crankshaft it should turn. Than check the crankshaft to see if it is bent.