As of writing the Youtube is down and it’s been an hour, Before Youtube never went down more than 30mints.
Youtube site and youtube apps are currently down and if opened they are broken not working perfectly.
Youtube has tweeted
Thanks for your reports about YouTube, YouTube TV and YouTube Music access issues. We’re working on resolving this and will let you know once fixed. We apologize for any inconvenience this may cause and will keep you updated.
Still we do not know what caused the site to go down, We hope that google will share details once the site is back online as was it a cyber attack or any server issue.