Small Engines (Lawn Mowers, etc.): yard machine wont turn over, drive pulley, model numbers

Question
I recently mowed the lawn and went over a foot long small branch from my orange tree.  Since then, I can hear the motor trying to turn over, but it won't.  I'm new at trying to fix stuff like this, so I wasn't sure how to even begin to look for what could be the problem.  I'm also afraid to turn the lawn mower on it's side to really inspect underneath it.  I'm not sure if it would cause oil or fuel to leak.  Can you help this novice?  

Answer
 Hello Teri:

 Send me the Model Numbers and Serial Numbers Off of the Mower and Off the Engine, Please. With these Numbers I can Look at a Diagram of the Mower and the Engine. However, if this is a Riding Mower; Remove the Belts from the Engine Drive Pulley. Try to Start the Engine. If it Turns Properly, then a Blade May be Bent or a Bearing May be Frozen. I can Assist you Better when I get the Model Numbers. Thanks.

 Respectfully

 John