I have noticed lately on My motorbikes that the rubber grommet in the shut off valve has gone bad because of ethanol in the gas. It plug up with rubber particles and won't let the gas thru. Just something I have come across on My bikes.
To start off you would want to check gas flow out of the line by taking it off and seeing if it makes a steady (if small) stream.
Then make sure your spark plug wire is on the plug right, some of them get bent up and make poor contact. Check the head nuts for tight. Then try starting it.
If no start or even trying to start, pull the plug and lay it on the head and walk the bike forward to turn it over and make a visible spark. If none, recheck your wiring connections and kill button.
Good luck to you.
These motor rarely have problems with the spark. If the flow isn't stead if you take the fuel line off, then that is your problem. If that not it it could be the float is sticking. There are only three thing you need to make run fuel, air and spark. if you have them three your good to go.