In message , Another John
writes
In article ,
Mr Fuxit wrote:

You've got the blade upside-down! The blade ends should point DOWN.

That was my first guess as well. Blades usually have an inscription
which indicates (or says) "This side to mower". It's often obscured by
years of mulched grass, so scrape the blade to find out.

Or turn the mower over (so you can see the blade), and give the 'on'
switch a quick blip so you can see the direction of rotation. The sharp
sides of the blade should be the leading edge.

As for a spanner: surely someone that uses this news group has a spanner
(or rather adjustable wrench) that will undo the nut? You need to block
the blade from turning, if you need to undo it: use a piece o'wood.
HTH
John

Be aware that the direction of the thread is sometimes 'opposite to
normal' (well, it was on one of my two mowers).
--
Ian