1. Check the fuel system. Make sure the fuel tank has enough gas and that the fuel lines are not kinked or obstructed. You can also try spraying some starting fluid into the air intake to see if the engine will start. If it does, then you may have a fuel problem.
2. Check the ignition system. Make sure the spark plugs are properly installed and that the spark plug wires are in good condition. You can also check for spark by removing one of the spark plug wires and holding it near the engine block while someone cranks the engine. If you see a spark, then the ignition system is working properly.
3. Check the battery. Make sure the battery is properly connected and that the terminals are clean. You can also check the battery voltage with a voltmeter. If the voltage is below 12 volts, then the battery may need to be replaced.
4. Check the starter motor. Make sure the starter motor is properly installed and that the connections are clean. You can also try tapping on the starter motor with a hammer to see if it will start the engine. If it does, then the starter motor may need to be replaced.
5. Check the timing belt or chain. If the timing belt or chain is broken or skipped a tooth, it will prevent the engine from starting. You can check the timing belt or chain by removing the timing belt cover and inspecting the belt or chain. If the belt or chain is broken or damaged, then it will need to be replaced.
If you have checked all of these things and the engine still will not start, then you may need to take it to a mechanic for further diagnosis.
