Removing the spark plug lowers the resistance to turning over .
At a guess, the grinding noise could have been the starter pawl grinding against the flywheel teeth
So start by taking off the blower housing and watching the starter when you try to crank the engine
The other possability is the ACR on the cam shaft has fallen apart so now there is no decompression .
The starters on mowers are deliberately weak so if there is no decompression most will not be able to crank the engine.
Check for decompression by removing the rocker cover and watching the rockers while turning the engine over by hand.
You should see one of them open slightly then close just before the piston is as far foreward as it goes ( TDC ) .
Each engine maker does this slightly differently so we are just giving general advice till you provide us with the make & model of the engine + its numbers.