Small Engines (Lawn Mowers, etc.): engine jamed, plug hole, spark plug

Question
I have a craftman 4.5  lawn mower and i can't turn over the engine. I hit something  hard and i was wonder if it could be that pin that is inside the motor could of broke.. how would i be able to fix that. Could i fix that myself.  

Answer
Remove the spark plug and see if you can spin the engine.  If you can't, leave the spark plug out, tip the mower with the spark plug hole facing up and see if you can turn the blade by hand.

What did you hit?  Is the blade bent or hitting the deck preventing the engine from spinning?

Let me know.
Eric