According to the Major, Xbox Live’s currently experiencing some billing issues.
From the man himself:
[Billing issues] means difficulties with account recovery, account management, Marketplace functions and/or making purchases. Users who already have Microsoft Points in their account will be able to redeem them. Until these issues are resolved, you will also be unable to sign in to Xbox.com, which includes posting in the forums and queuing up downloads on the Marketplace.
Matchmaking, friends lists, messaging, and so on, are still good to go and unaffected at present.
Engineers are working on resolving the issue, so keep watching the status page in the support section of XBL for a resolution.