Life is a random, cruel, strange, wonderful, enigmatic, crazy thing. It mostly comes down to time. By that I mean commitments on your time - school, work, friends, etc; and what time you need for yourself - rest, relaxation, etc. As you get older, or at least so I've found, more things try to take your time. You are simply less capable of doing something for hours all the time because too many other things try to get in the way. So it really isn't a surprise when changes in someone's life makes them have to drop your game. But, what does that mean for the game itself?
This is the worst result, but a player leaving the game can be game over. This normally happens when it is the GM who has to drop, but can also happens with players. Maybe the player is someone's ride, maybe they're the host for the game, or maybe the game is just so small that it really can't work with one less person. In this case it can be best to just acknowledge that, try to find a way to resolve the ending, and then move on. Maybe another game, maybe a different game, or maybe something else. Either way, a quick break can often be best. Lingering can just lead to frustration and annoyance.
Much happier than the previous, sometimes - usually - it is just some plots that get dropped. This is no different than if the character had died and you were simply waiting for the PC to bring another character into your game. This can be its own frustration and annoyance though because sometimes a lot hinged on a particular character being in the game. Sometimes the current plot hinged on that player's personal plot to happen properly. Sometimes you've just spent a whole ton of time working things to make the player come to the fore and have their time and they're gone. In my experience, one or all of these is almost always the case when someone drops - no matter how justified the reason - simply because that is how life works. However, there is more that can happen...
Restructuring The Group
Ever notice how often the people with the most going on in life are also the ones who are the most active in game? I mean, yeah, it can be different in groups but a lot of times the people who like to keep moving, keep doing things, and be interacting tend to be the ones who speak up and act a lot in game. This, in game terms, means that they were also likely someone of relative importance to the group itself. Maybe the leader, maybe the face, or maybe the girl with the contacts. Either way, they left and they're leaving a hole. Even if the person isn't active they are leaving a hole in the group. Were they the mage? Now your group doesn't have a magic user, or has less magical functionality. The fighter? Rogue? Cleric? doesn't matter, their character had a role in the group and that role is now empty.
As the GM this means you need to be aware of that, and likely go back through planned encounters to address the change in abilities. It isn't the players' fault someone had to join, so you don't want to screw them over when after the fire mage drops game they keep running into things that need fire to be defeated. It's just not cool. As the players you need to figure out what the hole is, where other holes may be, and then find ways to deal with them IC. Maybe there are Out Of Character ways to do it, but in character you need to address the issues too.
The Dropped Character?
Perhaps the most obvious but least thought of factor is what do you do with the newly abandoned PC? Do they just become an NPC? Do they vanish from the realm? Do they leave the city? Leave the group? Get arrested? Do you deal with them IC or OOC? What about their belongings? What about their relationships? What about favors owed to them or by them? These are all potential hooks you can use to make the game more interesting. You just have to factor it in when someone leaves.
So, what do you do? :)