g'day monk,

could be a number of reasons, but first you can try some things.

check oil level make sure it isn't overfull.

take out the spark plug then pull starter see what happens, if oil has
gotten into the upper cylinder it could stop the piston travel, so
doing this you may shoot some oil out of the plug hole, this is a
simple fix just pull starter a number of time to blow as much oil out
as possible give plug a bit of a was off in some fuel then replace all
things being equal the mower should start though it could blow some
smoke for a bit.

if after taking the plug out you can't turn the motor by the starter,
tilt the mower in the case of b&s with the carby side down or the plug
end up and try turning the motor by turning the disk. if you can turn
the disk then this could also mean the starter is faulty if you can't
then you could ahve serious problems most likely a bad score in the
bore from running very low in oil.

if none of this helps take it to a mower repairer for their advice
opinion.

see how you go

len

snipped
--
happy gardening
'it works for me it could work for you,'

"in the end ya' gotta do what ya' gotta do" but consider others and the environment
http://members.optusnet.com.au/~gardenlen1/