Almost guaranteed you have stale fuel as suggested b John - if it's been in there all winter, it will pretty much non-combustible as far as your mower is concerned.
phil_saunders(a)bigfoot.com
Phil Saunders
April 2009
Make sure there is fresh petrol in the tank, and that the fuel tap is on. Always try to start on full choke. However, if you've been cranking the thing for a while, the spark plug could well be wet with excess fuel. Take the plug out, crank the engine a few times, dry the plug and have another go.