If you are having issues signing into Xbox Live on your Xbox One, Xbox 360 or even outside of those consoles, you are definately not alone. Microsoft has admitted that it is currently dealing with sign-in issues with its online gaming service.
The Xbox Live status page currently has the following message:
"Xbox members, are you having issues signing in to Xbox Live? Our team is engaged and investigating this issue right now and we will not stop till everything is running smoothly again! Thanks for your patience while we work. We'll update you when we have more information."
We will stay on top of this situation and let you know once the services are back up to speed.
Update: According to the Xbox support page, it looks like things are all in the clear once again. Happy gaming!
Thanks to Christoph for the tip!