The default apps that come with iOS devices like Safari, iTunes, App Store, Camera, Mail, YouTube and others can not be deleted. If you find any of these apps missing or settings grayed out you may have restrictions enabled.
To see if restrictions are enabled go to Settings-> General-> Restrictions. If you are asked to enter a passcode you have restrictions enabled. You will then need to enter the passcode or Restore your device in iTunes if you have forgot it.
If you know your passcode, enter it and disable Restrictions. You should find the app if restrictions was causing the app to disappear.
Another possibility is the app is on a different page than the Home screen or in a folder. If you still can’t find the app, you can search for it by swiping from left to right from the Home screen then enter the name of the app.
If it is an app that was installed from the App Store you can download the app again without being charged as long as you are signed in with the Apple iD that purchased the app.