engine won't start, and is making noise?

My mower was idling in the middle of the road when i heard a pop and then the engine stopped cold. Try restarting it but it wouldn't start and while trying to start it the tractor vibrates and i hear a knocking noise. It acts like something is loose and/or broken but im not sure what it is. Occasionally i get a backfire from the muffler after about 3 attempts at starting it. The flywheel free spins by hand, but the last time i tried starting it, the flywheel locked in a certain place and then wouldn't move so i tried starting it again and then it free spun again. When i took the cover off and watched as someone tried starting it nothing looked wrong just the noise and no starting just spinning. my engine is a Briggs and Stratton 12hp. I need some direction and will take whatever i can to try. thank you