Providing you have checked the obvious, fresh fuel in tank!
Remove plug hold against an earth. Check for spark by pulling starter.
You have no spark?= faulty plug or coil assembly or cut-out circuit not open.
You have spark?= fuel problem or low compression (requiring further tests):-
refit working spark plug. Next check air filter and remove it. Clean foam in petrol and dry it, blow out on airline. Soak filter in wd40 and squeeze excess out. Or alternatively replace filter later on.
With air filter removed spray "easy start" in intake. Attempt to start it. If it fires then carb is probably blocked.
Remove carb, completely strip, clean and blow out on airline. Refit with fresh gasket.
Allow carb to fill with fresh fuel. Prime carb and start!
Tom The MotaMenda, April 2006