Instructions
Verify that the publisher still runs the game's servers. A game's publisher can shut off the servers at its own discretion, and it may never choose to turn them back on. For example, nobody can use online features for "Facebreaker" by Electronic Arts as of January 2011. Visit the publisher's website and see whether it still supports the game.
Try connecting to the server when there are fewer people online. It can be difficult to connect to the server on weekend afternoons as opposed to weekday nights.
Move any obstructive objects from between your router and your Xbox 360 if you have a wireless connection. This includes metallic objects and cell phones.
Troubleshoot your connection through "Network Settings" under "My Xbox." The test will tell whether anything is wrong with your connection.