If you are using the default app drawer, it is possible that you have hidden it. I am using Nova Launcher, so I do not recall exactly how to show the hidden items. It should be one of the choices when you press the menu button when you are in the app drawer.

 

I wish I knew what to tell you. Mine has never disappeared from apps.
As a matter of fact I did an update, I believe last night and no problem.

Here's a suggestion- go to Youtube via Google Play. See if it says "open".....if it does, it's on your phone somewhere. If it says "install" then somewhere along the line it was removed.

Other than that, I'm out of ideas.