Fuel, compression and spark. Does it have all 3? Make sure you have good spark. Your on/off switch could be faulty. Try unplugging the yellow oil level low wire also near the on/off switch. If the engine is low on oil or if you tip the engine, then the low oil sensor kills the ignition. Unplugging the sensor temporarily will help diagnose your problem.
Do like was suggested, squirt some gas in the carb or down the spark plug hole and give it some pulls, see if it starts for a few seconds. If it does them you know you have a fuel issue. Where you "adjusting" the rocker clearance for the valves? If you have them to tight you won't have any compression and it won't start.
Explain your break in procedure and perhaps we can help you get it running again.
Danford1