Is there fuel in the tank? Sure, you know there's plenty in there, but play it safe, pop the cap and take a look. If you can't see it sloshing around, the tank might be empty after all. Of course, on bikes with underseat tanks and remote filler caps, this check won't be possible. If the bike has a petcock, an item that seems to be found less and less these days, place it in the reserve or prime position and try starting the bike. Still won't start? The two most likely suspects: Either fuel isn't reaching the engine (assuming that the tank is full), or you've lost the spark. If the bike shows absolutely no inclination to start, I'd head for the spark plugs. If the bike coughs and sputters, I'd lean toward the fuel delivery side. But don't forget, fouled spark plugs will certainly make a bike hard or impossible to start, and may mimic a fuel delivery problem. Actually, since I'm pretty lazy, I always check the easiest thing to reach first.