In what seems to be par for the course when a big game launches, Xbox Live Core Services are on a limited status, meaning that you'll have problems signing on. Now, this could totally not be related to Titanfall coming out today, but it's still not one of the better things to experience at this crucial time.
You can hit the Xbox Live Service Status page to keep up to date on how things are going if you don't want to keep trying with your Xbox One. Let's hope it gets resolved soon enough. But hey, if you can get into Xbox Live, it seems other things work just fine according to the status page.
Update: Major Nelson has confirmed that the issue is not related to Titanfall.
Update 2: As of 11:00 EST the services are back online. You may need to power cycle (hold the on button for ten seconds until it powers off and then disconnect the brick for a minute) your Xbox One to be able to login.