If you're pretty knowledgeable with how things work in most internal combustion engines and mechanically sound, then I would suggest the following but be extremely cautious!:
If you've checked all the stuff the other riders have suggested and also see if there's anything else easy to check like obvious switches, vacuum hoses out of place, etc (trust me, you'd rather have it be that). Then you can take a look at my suggestions as this is what I would do next given my experience with cars, but you must be careful.
Engines need the following to run: Intake, Compression, Power (fuel and spark), Exhaust so let's start ruling out what it's not getting.
I suspect that we're not getting one of the following two (fuel and spark): - Also please, please, please, be careful.
1. (Are we getting fuel?) Be cautious as to what you're about to do: Take a spray bottle and fill it up with gas, remove the intake housing or any way you can get exposure so that you can spray full in the engine without prohibiting any functions required for the engine to run. Spray the fuel in and try starting the bike like you normally do. If it runs, then it wasn't getting fuel or enough of it, then we can go from there.
2. (Are we getting a spark) Again be cautious with this one as it can be a bit more dangerous. Take off one of the plug wires off from the spark plug and find a decent size (any head will do) screw driver. Carefully put the screw driver in the the housing where the spark plug would normally go. DONT shove it in there, all we need is for the metals (theres a metal clip in the wire that snaps onto the plug) to touch each other, the slightest contact will be all we need. Next find an area within reach that we know can be considered ground (usually and metal part). Aim the flat blade so that its parallel to that surface while still having the plug wire "connected", hover it over that area but don't have the two (metal and screw driver TOUCH, again don't let them touch) Now I recommend wearing rubber gloves for safety and using your right hand only (farthest from your heart). While holding the plastics/wood part end of the screw driver while its next the place you found believing it to be a spot for ground, try and crank the motor while watching to see if there's a spark. If we see a spark then we know that a spark is being generated, if not then we can go from there.
If we're getting fuel and we're seeing a spark then I'm not sure where else to go right now, we can have the plugs tested/replaced but I haven't really ever seen all plugs go bad at once.