I would start by spraying a little bit of starting fluid in the air intake.
Try starting the bike after spraying the starter fluid.
-If the bike starts up and then dies, then you have to check the fuel filter, clean the carburetors, and check for clogged fuel lines
-If the bike cranks over without signs of starting at all then you might have an electrical problem with your ignition system. I would check the coils on the spark plugs and the spark plugs themselves.
Good luck