when you forget to turn off the gas, most carbs will overflow because the bike leans too much on the kickstand - the overflow will go right down into the bottom of the motor (and these don't have a drain plug on them)
usually, you can get it started with enough effort and it will run poorly until all the gas is pumped up into your muffler (do that a lot & the muffler can become clogged)
if you can't get it started, you may have to take out the spark plug, turn the bike upside down, and turn the motor till gas stops coming out