Start youtube video after activity restart

I've got a problem with youtube player in android. I made a button to start playing youtube. When I start the app, it works good, when I press it video start. But when I close the app, back to the app and after that I click the button nothing happens. The only way to start video is to tap on the basic you tube play button. How can I do that with my button? I am using YouTubePlayerSupportFragment. After restart activity, when i try to make "youTubePlayer.play()" in debug mode it says to me, that this operation is undefined. I don't know what to do.