Try engaging the foot brake, then pressing the blades button "off" while you turn the key (whilst sitting on it).
Failing that clean the battery connections and retry using the above steps (as it engages all the safety mechanisms).
It sounds like the switch as mine used to do the same thing. Hope it helps
(If this still doesn't work you may have a hydraulic engine. This simply means that the petrol has seeped into the head and the piston cannot compress it. If you remove the oil dipstick and it smells like petrol, remove the spark plug, try to turn it over then drain and renew the oil). Try the simple one first !
James
September 2008