Oftentimes if you notice an app from the App Store has gone missing, it will still be on your iPhone, only it may have been hidden if you adjusted the Content Restrictions on your device. Apps such as the App Store, FaceTime or other apps that are included with iPhones will be turned off when you choose particular content restriction filters.
Other apps that might seem to be missing from your iPhone, but are actually hidden other than the App Store and FaceTime include Safari, YouTube, the camera, Siri, iBooks Store, and AirDrop. Along with apps, functions and features can also disappear when particular content restrictions are applied such as the ability to install and delete apps, location services, adding or modifying email accounts, the ability to make in-app purchases, options to change privacy settings, along with other functions and features which can be restricted if you set the restrictions for your iPhone.
You can get the apps, function and features to show back up on your iPhone by adjusting your content restrictions back to being unrestricted. To do this, go to Settings, General and then scroll down until you see Restrictions and tap on that. You will need the four letter passcode that was set when the content restrictions were originally set up. If you don’t have that passcode, you might need to restore your iPhone to its original settings. This means you will lose data, however you will be able to get the included apps, features and functions back. Then, if you have a backup of your other data, you can restore that after you have completed the restore process for your iPhone.