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, 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 the tank is full), or you've lost the spark.