Usually when one of my players cancels, we play anyway. But if more than one or two cancel, or if it's an important session like you said, then I would run a different game. We always have a couple on the back burner for such a situation, because it sucks making new characters all the time.
For example, right now our main game is Eberron, but if not enough people show up, we do D20 Modern with zombies or Rappan Athuk.