Toubleshooting Connection
If you cannot connect to a game you trying to start, verify that your game is running.
If you were suddenly disconnected from a game, the last message from the game in your client may tell you why, the game may have gone down unexpectedly, or some part of the route between you and the game may have become temporarily interrupted. Sometimes, these interruptions are caused by a NAT or firewall.
If you simply failed to connect to your game you normally use, many of the above causes are possibilities, but it is also possible your connection is down or your DNS is temporarily experiencing trouble.