Vacations. Family Emergencies. Work. Internet Issues (for VTTs and G+). There are many possible reasons for a player to miss a game.
What happens with the game when one of your regulars can't make the session?
Do you play something else? Back in my college days, if we were short a regular we would often pull out Chaos Marauders, Ameba Wars, Nuclear War, Risk or Talisman. Occasionally if someone had something prepared for a different RPG system we might do a one shot.
I know some groups (as we did on occasions) bring the absent player's PC along as an NPC. The problem with this method is no likes to return to their PC being dead, which does happen.
Depending on where the last session ended, you can leave the absent player's PC back in town, but it was rare for any of our sessions to wrap up that completely. Usually we were halfway into a dungeon when we wrapped up for the night, with the intention of picking up from there the next session.
So, how do you handle missing players in your campaign's game sessions?