Three Big Goals
In a lot of ways the first session back is a lot like the first session of a game. As the GM you should have three goals in mind for your session, and try to prepare the game to let you address these three things. The goals?
- Get your players back into their characters
- Get your players back into the world
Back Into Character
A long break can cause people to lose characters, or to at least lose track of characters. As such it is important to give people an opportunity to get back into character. Room for RP, room to look over who the character is, room to maneuver things, and room to dip toes back into the mechanics of the character are all important. Try not to worry about plot as much as you do about relationships and reinforcing them. If someone is playing a thief, have them find a wanted poster for something they did. A warrior could run into an old comrade. Small things that help remind the character of the role they play, why they play it, and why they're awesome at it are all good things.
Back Into World
You also have to reintroduce the world. This is tricky because the PCs could be somewhere where that is awkward. Try to ease people back into the world though. Don't hit them with the main plot right away, but instead hit them with aspects of the world. In D&D it could be good to let them hear gossip or stories in a tavern. L5R can be re-established with some display of how the world is Honor vs. Dishonor instead of Good vs. Evil. Modern worlds can just be established as normal, and then give signs of the abnormal to help guide the players back.
Once the game is going and the players feel the world again you can worry about other things. First, just get them back into the normal swing of things. You can disrupt that later.
Fun is Key
A lot of games break after hiatus because people don't have fun. They don't know it's because they're not having fun. They think it is something else: they lost the character, they lost the world, they forgot the plot, they can't get into the game again, etc. You can avoid this by just trying to make things fun. It's one of the reasons I try to not focus on metaplot in a first session back but instead just on having fun events happen. Give the PCs some chance for competence porn (i.e. when they get to look super badass.) Put some funny moments in. Give moments to shine. Tone down defeats and moments of despair. Just have a good time and let everyone in on it.
Turbulence is Fine. Get the Ball Rolling
Finally, as a heads up, don't expect the session to be smooth, even, or to flow properly. People are going to fall in and out of character. You are goingto fall in and out of GMing mode. The world is going to be explored and joked about. All of this is fine. Your job isn't to run a smooth session. However, you do need to get the ball rolling.
The point of this session is to overcome the inertia of nothing happening, and to get it so people are in world and experiencing things.
The biggest rule is to have fun. The next is to get the ball rolling. Good luck!