This article explains how to get apps back on an iPhone if they've disappeared. Instructions apply to all iOS devices.

Not all apps that appear to be missing are actually hidden or gone. Check to make sure the apps are truly missing and haven’t simply been moved to another screen or folder. After iOS upgrades, apps sometimes get moved to new folders. If you recently upgraded your operating system, try searching for the app you’re looking for using the built-in Spotlight search feature.

If Spotlight doesn't launch the app you're looking for, check to see if Restrictions are enabled in the Settings app. How you turn them off depends on which version of iOS you're using.

Your apps could also be missing because they’ve been deleted. Starting with iOS 10, Apple lets you delete some pre-installed apps (though technically those apps are only hidden, not deleted). Earlier versions of iOS didn’t allow this.