It's beginning to become something of a mantra: Xbox Live is once again suffering from connectivity issues. Recently, it was signing into the service, accessing Backward Compatible titles, and purchasing content that wasn't working. Right now, Xbox Live users are once again having issues with signing in, and we'll have to wait and see if hackers are once again going to claim responsibility.
As always, check the Xbox Live Status page to stay current on how well the service is working for you. Sign into your Microsoft account and give a phone number, and the company will ping you when the service is back up.