Small Engines (Lawn Mowers, etc.): lawn mower blade spins freely, lawn mower blade, briggs stratton

Question
I ran over a rock with my old 3.5 briggs & stratton push lawn mower and now the blade spins freely.  I removed and retightened the bolt that holds the blade on but that didn't help.  THe shaft feels normal and engine seem to run ok.  Any suggestions would be grealy appreciated.  

Answer
The blade hub, if equipped with only one retaining bolt, should have two shear pins to keep the blade from spinning yet allow shear if something is hit.  You'll have to do a visual inspection of the hub, blade, etc., to determine what you need (as there are hundreds of variations depending on mower brand).  Hope this helps.