Update: All clear! If you were having issues signing in before, you should be able to now.
If you're having a bit of trouble signing into Xbox Live this afternoon, you're not alone. Microsoft has acknowledged that Xbox Live is currently experiencing some issues, causing some to hit a wall when trying to sign in.
From the Xbox Live service status page:
We are aware that some members may be having problems signing in to Xbox Live. We are working to resolve the issue as quickly as possible, and we will be sure to provide you with frequent updates.
This applies to Xbox Live across all platforms, including Xbox One, Xbox 360, Xbox on Windows 10 and other devices as well. Outside of signing into or managing an account, it appears that other services are unaffected.
Hopefully Microsoft is quick with a fix, but if you were hoping to do some afternoon gaming, you can keep an eye on the issue at the Xbox Live service status page for more.