If you are trying to sign into Xbox Live right now then you may be having some trouble, in that you can’t do it. Microsoft has acknowledged the issue and have said they are working hard to resolve the problem though at this point it’s not clear exactly what the problem is or when it will be fixed. At this point, they’ve advised Xbox Live users to continue watching their status page for updates.
We’ll update this page if we get any new info.
We’re working hard to resolve the current Xbox LIVE issue, all. Check in here: xbx.lv/r0tXNK for updates! ^KE
— Xbox Support (1-5) (@XboxSupport) April 13, 2013