Need help with Craftsman Briggs Stratton 15.5 Spins but wont start after it quit.

Sorry about that long title, here's the story. I'm mowing the lawn and I hear a fwump like the blades caught some dirt then the mower dies and winds down. After that it will not start only spins and does that wind down thing... (see video). I have done a visual on the flywheel key and it is still in place. Also one of the blades was loose (eek) when I looked under the deck. (tightened that right up) any ideas would be greatly appreciated...

https://youtu.be/1rUeLhgaiGk