There are several things you can do. Most of them are not too great for your game, but unfortunately, people who don't help the group succeed are often more detrimental than I think they realize.
Aim for a cooperative solution
The first thing I'd do is send them a message explaining why their absence is a problem. If they're new, explain that an RPG requires some continuity, and people usually can't simply leave. If they're accustomed to tabletop RPGs, then you may want to emphasize the game itself. Be sure to phrase the question like you're asking them for their help. In general, if you phrase your questions and interrogations like you're asking for assistance or need others' help, they will be much more willing to help.
What I mean by this is: phrase your questions like you want to work with them to figure out what's going on, because ultimately, that's the better solution. Aim to work something out with them. Chances are good they'll come back and say "You know, I really just can't make it to those sessions. [Can we change the time around a bit?/Sorry, I'll have to drop out of the game.]"
If you can come to a cooperative consensus, you will prevent the dark cloud which has the potential to seep into the table and sour gameplay. The goal here is to make them aware of the problem, and open to coming up with a cooperative solution. If they're not both aware and willing, anything else you do won't go over well.
Oh, and one last thing: Try not to sound bitter about it. I know it's difficult, and I do it myself more often than I'd like to admit, but it actually doesn't accomplish much. You really do want to aim for a cooperative solution.
Actually resolving the issue
If your player still would like to play, 'tis time to think of a way to make it happen. Take a look at the list on As an unreliable player, how can I lessen the burden on my group?
Your ideas may differ, but here are the things I'd do:
Ask them to let you know when they're going to be gone, as far ahead of time as possible. As a GM, it's awful when you have a great, fantastic plan, and someone doesn't show up. You and I've both had this experience. To minimize this, it's helpful to know when a person is going to be absent.
Plan your campaign so that they're not critical to the storyline; make sure they know and understand why this is. You can't depend on somebody who's not necessarily going to be there. As such, you will have to plan around their absence. This will result in them being in the spotlight of the game less often. To prevent them being upset about this, make sure they understand why this is the case.
Ask them to provide reasons why their character might not be able to show up. In order to prevent random discontinuities, ask them to come up with reasonable excuses for their character's absence from play.
There is just one other thing to point out: If, at any point, they become antagonistic towards you, don't entertain them. Obviously the first thing to do is take a look at what you're saying and see if it prompts that kind of behavior. If it doesn't, however, then you are by no means obligated to entertain somebody who will be actively harmful towards your game.
These things should help you alleviate the tensions during gameplay caused by absent players. Hopefully this helps you come to a solution for this problem!