It depends on the time of year that this is happening as to whether or not it can be a fuel or mechanical issue.
If it is the first time you try to start the push mower for the season, then it most likely is a fuel problem. If it is during the season, then it could be a fuel problem or a mechanical one.
Always check the fuel first to see if it is good or may have some water in it. If the fuel is good, then check for spark and to see if you are getting fuel into the cylinder. The spark plug will have a slight wet appearance if fuel is getting into the engine.
By narrowing these items down, you are troubleshooting them one by one to find out the cause of the problem. Once you have narrowed it down to the component that is causing the problem, change it out or adjust it and retest to see if it resolved the problem.
If you need help with your push mower not starting, visit our Locations page for a lawn mower repair shop near you.