Lawn mower unused for the winter will not start Changed oil and gas spark at spark plug what else should you look at and how do you fix it?

If you left any fuel in it before you put it up for the winter, the old gas has turned to "goop" and stuck to the walls of the fuel line, carburetor and probably the rest of the fuel system. You will have to disassemble and clean with carb cleaner or similar product (also clean the fuel lines). When the mower is put up till the next season, it is wise to run the engine until all the fuel is completely gone and not any left within the fuel lines.