Try out the battery contacts for any loosely fitting terminal and tighten it. Follow the electrical wiring and ensure the connections are not loose nor detached. Turn the crank shaft belt by hand if the engine picks slowly and feels too heavy to make the engine light and try igniting again.
Ensure there is enough fuel in the gas tank by looking at the "low" and "high" levels mark. If it's below the mark, add fresh fuel that has not been stored. Check if the engine is a four cycle and the oil used is the right type. Look for the fuel shut-off valve and the fuel vent cap and ensure it is open and fuel is flowing freely but not flooding the carburetor.
Check all the safety mechanisms to make sure the "lock" mode is off. Some manufacturers include safety mechanisms by adapting the vehicle engine to start after certain conditions are met before igniting. Disengage all transmission attachments then engage the ignition switch to an "on" position.
Pull out a spark plug, attach the hexagonal head against the bare metal body of your engine, start the engine and check for a blue spark from the spark plug. Adjust the spark plug gap to 0.03 inches or replace spark plugs if the flame is not blue. Replace the plug back to its position if it's working and start the engine again.
Try the spark plugs on another machine. If there is still no proper spark, then replace the plugs. Disconnect the module terminals that connect to the kill switches and ignite again. If you see any spark, then it means the switch or a wire is defective. Check the flywheel timing mark to see if it is in its proper position and the connecting gear from the starter to the engine is not worn out nor displaced.
Look for air and oil leakages in the linkages and gasket seals by using a wrench to tighten any loose bolts and nuts on the engine block. Start the engine again and observe the areas for any bubbles. If it's OK then disassemble the fuel line and clean it step by step. Remove the filter and replace if needed or return it to its position.