Don’t overthink the problem, check the simple things first…
Was the ignition switch in the “on/run” position before the starter button was pushed?
It may seem obvious but the first step is to make sure your ignition switch is in the on/run position.
Is the kill switch (usually on the handlebar adjacent to the throttle) turned to the “on/run” position?
The position of the engine kill or cut off switch is one that is often overlooked. Many people don’t use it regularly so when it is used or someone else uses it, we forget to turn on the switch. If it is off, turn it to on or run.
Is the kickstand in the fully retracted (up) position?
Some motorcycles require you to fully retract the kickstand. In those cases, a stand safety switch may not allow the starter to engage and crank the engine.
Is the clutch lever pulled in all of the way?
If not, a clutch safety switch may not allow the starter to engage. You should put your motorcycle in neutral and try again.
Is the battery in a sufficient state of charge to crank the engine?
If not, have the battery charged or replaced as necessary. You can check your battery by using your horn or headlights, if they don’t work then you probably have a battery problem.
If the starter engages and cranks the engine but it fails to fire, is the fuel valve (if so equipped1) in the “on” position?
If not, turn the fuel valve to the “on” position, wait 15-20 seconds for the carburetor's float bowls to fill and try starting it again.
Is there enough gas in the fuel tank?
You may be certain you had fuel, but it’s worth a look. If possible, take off the fuel tank cap and look in to make sure there is fuel. If not, add fuel.
Is the engine carbureted or fuel injected?
If carbureted, was the choke or mixture enricher set to the appropriate position to start the engine at the current temperature? (Both ambient air temperature and engine temperature need to be considered.)
If all of these items have been checked/corrected/adjusted and it is still not starting, additional diagnosis and tools may be needed.