As of this moment, Xbox Support has said that users may not be able to sign into Xbox Live. Their support page says that multiplayer gaming, store and subscriptions, account & profile and cloud gaming are all down.
We are aware that users may not be able to sign-in to Xbox Live at this time. Our teams are currently investigating to fix this issue. We will update here and on https://t.co/PzAdjUFMJj when we have more information to share!