Sometimes the Xbox service goes down which can cause errors in-game. To check this, go to http://www.xbox.com and log in with your account. Click on Xbox Support in the top menu and then Xbox Live Status.
On the Support Page, you will see a link near the top that says "Xbox Service is Up" or "Xbox Service is Limited" or "Xbox Service is Down". If service is limited or down, you may not be able to log into your game or play as expected until the service is fixed.