Early on Thursday morning YouTube went offline. At this time it is unknown the exact cause of the service being unavailable but at the time bookmarked YouTube videos were still accessible and direct video links.
There are many possible causes as to why the service could have suffered the outage, a botched upgrade, a DDOS attack, or possible even equipment unexpectedly becoming faulty along with the backups failing too, as Microsoft experienced not too long ago.
While the cause is not currently known, there is some speculation that it may be related to Chinese hackers but at this time it is too early to determine the cause of the downtime. We're awaiting a comment from Google spokespeople on the downtime.