How to fix the blade of lawnmower?

35 Classic Briggs and Stratton petrol lawn mower. the blade just dropped while mowing the lawn. theres a small piece of metal that fell out of the middle too. we managed to refix but when we mow the lawn, it spreads the cut out grass around rather being picked up in container at the back. we checked in the manual and everything is fiex as per instructions. any ideas what could be wrong or what could be done to get it in right working order?
ManyThanks

Mr Vinning
April 2008