This is one of the most frequent problems with a lawn mower and especially if it has sat over the winter. Depending on where it stayed over the winter could add to the problem. If it was in an unheated shed for example, the battery could have run down or mice may have chewed your ignition wiring.
In many cases it is the fuel itself that has gone bad. Over time fuel becomes stale and the lighter part of the fuel will evaporate which makes it very hard to ignite. The fuel will then begin to phase separate into different parts and dry up which plugs up the ports in the carburetor. This will cause your lawn mower to not start. The only correct way to resolve this issue is to clean the carburetor and properly dispose of the fuel in the gas tank.
Your ignition system could also be the problem for your lawn mower not starting. Your ignition coil could be bad or the spark plug as well. The kill system could also be malfunctioning causing a no spark condition. A key switch or safety switch could also be the culprit and by testing them out with an ohmmeter, they can be ruled out if they are in working order.
Those are the main things that cause a lawn mower to not start when it should. By inspecting and diagnosing them one at a time, you can rule out which items are good and which are not.