It has been verified that Google experienced an outage which affected a vast majority of its user and services on Monday, December 14. It is a rare occurrence and has been verified by the workers at Google.
YouTube, Docs, and Gmail were in rampant use by Google browser users all over the world that suddenly all of them went offline. Play store and Google Drive services also completely went offline for many users.
A hashtag #YoutubeDown started trending on Twitter where all users were comparing data and wondering what was the case which was verified later on by Google. They state that most of the users are able to access the browser and apps again by now.
Some users were able to access YouTube through incognito mode hence it seems the outage was related to the authentication process as users were unable to sign in and out of their accounts. The problem was fixed within a couple of hours.