Model 407777 type 0167e1 starter spins and will not engage?

I placed my mower in idle after running for about an hour of mowing to open a fence (blade was disengaged). Just as I got the gate open, I heard a pop and then the mower shuttered. it now will not start, the flywheel spins and spins but does not engage to power the unit. What items should I rule out first in the process of elimination to get this running again?