Normally 2 things cause a bike not to start. Spark or Gas. If the bike is not cranking it's an electrical problem. If it cranks but doesn't start, check for a spark. If you have a spark it's a fuel problem. Take the airbox off and see if there's fuel going in the throttle bodies as you try to crank it. If you have spark and there's fuel in the throttle bodies, it could be bad gas. Very common when something sits for a long period of time. Drain and refill the gas tank.
These suggestions are for a bike that was running fine the last time you started it. If you did any work on it and it won't start, you need to re-check your work.