PLEASE help!

2003 Weeres 20' Pontoon Boat has a 40hp Mercury Outboard.

It ran GREAT! Idled GREAT! Started GREAT! EVERY TIME..

One day, I started it (it always started on a dime and ran like a
dream), drove it a few hundred feet, turned it off. About 2 hours
later, I turned the key and ABSOLUTELY NOTHING happened, no sounds, no
nothing. (battery is perfect)..

I called a 'boat mechanic' who said 'pull the cover off, I'll be right
over'. He proceeded to BANG on my flywheel about 15 times, on the side
and here comes grit and pieces of the magnets.

My question: Is there ever a time you can ever bang on a flywheel
besides the striker plate or during the removal procedure?