There are only three reasons all engines will not start. No spark, No fuel, No compression. Now if you are asking what are the causes for one of these conditions, then you must first find out which of these conditions is occurring. No spark could be any of the following; bad plug, bad coil, bad switch, shorted wiring, etc. No fuel could be any of the following; bad fuel, bad pump, air leak, dirty carb, etc. No compression could be any of the following; bad rings, bad valves, bad valve clearance, blown head gasket, etc. You must find out which reason is causing the problem and then diagnose which of the causes is creating the problem.