Small Engines (Lawn Mowers, etc.): motor stopped wont restart, mtd yard machine, pistion

Question
The mower is a MTD Yard Machine 4hp B&S push mower. Was mowing a tall area of grass and mower stopped quickly and wouldn't restart. Checked for spark, fuel. When I turn the blade it turned very freely, trought maybe the key broke on flywheel, but flywheel turns when blade is turned. Took out spark plug and piston still goes up and down. Covered plug hole with thumb, cranked over, didn't feel like I had much compresstion. Any ideas? Thanks, John

Answer
Is the oil level correct?  If the pistion still goes up and down and the oil is fine then you should have compression.

The flywheel key may still be sheared.  A sheared key throws off the ignition timing.  

Have you checked the key?  You will have to remove the flywheel nut to check.

Eric