When you open the launcher, it connects to the login servers and generates a session ID in exchange for your user login details. Then, whenever you connect to a game server, the login server matches the last session ID it got when you logged in to your game's session ID. However, these session IDs are only for one use (hence "session"), so they expire after a certain amount of time.
To get a new session ID, simply close all instances of the game and the launcher, then start the launcher again.
I know restarting the launcher works, I have to do it occasionally. However the session ID explanation may be somewhat incorrect or out of date. Feel free to correct.