Glad you found that it was the spark plug. I advise people to start with the cheapest fix first...
#1 replace the fuel!
#2 replace spark plug etc etc
Of course you just said this yourself but it's exactly where people should start... oh wait... FIRST check that the ignition switch is in the "on" position. No joke - a friend of mine couldn't start his old bike and I just simply switched the ignition over the other direction and it started right up.