why is blade not turning?

update to earlier answer. look underneath and see if the blade spins on it's own, if so then it's the bottom key way broken.
unscrew bolt and pull off unit, these can be extremely tight if they have not been removed for a long time so a puller may be needed.
also check if it is direct drive or does it have a belt driven blade as this could have come off or broken..

cg
May 2017

HI
you will probably find that the woodruff key has broken , normally situated under the fly wheel.
fly wheell is on top of the mower, undo nut and it should pull off and the key is in between fly wheel and main shaft. it is D shaped and fits into a slot on the shaft.
easy fix.

cg
May 2017