This is an all too common problem for lawn mowers today. The ethanol blended gasoline that is in use today is the culprit.
Your lawn mower sits for 5-6 months out of the year with gasoline in it and then you try to start it in the spring and nothing. Today’s gasoline begins to go bad after 90 days. It will then begin to separate the different chemicals all the while the ethanol is attracting moisture into the gas tank through the humidity in the air.
As the different chemicals separate from one another, they begin to dry up inside the carburetor. When this happens, the idle and main jets become plugged up as well as the emulsion tube. Now gasoline cannot go through them and into the engine, so that is why it will not start.
The carburetor will need to be disassembled, cleaned and put back together. If the ethanol intrusion is really bad, the carburetor will need to be replaced. It could be a very expensive one or an affordable one depending on what brand of engine it is.
It would be best to take your lawn mower to a shop near you to get it repaired by someone skilled at carburetor cleaning and assembly. You can find one on our Locations page.