OK So you had been running YouTube. The tablet shouldn't show that if it was actually turned off and not just gone dark due to not being used. I would suggest you turn it completely off and then back on. It should resolve the issue.
If it doesn't, then go into "Settings" and then "Apps". Scroll over to the "ALL" list and then scroll down to "YouTube". Tap it and then table "Clear Data". Once that is done, again turn off and then back on the tablet and you should be good.