Hi
When looking for the Mail app's icon, keep in mind that you may have moved it to a different Home screen and/or moved it into a folder (which may itself contain multiple pages of apps):
To search for the app, when viewing the Home screen on your iPhone:
Swipe down on the centre of the screen > in the Search box at the top of the screen, type "mail" > if the Mail app icon is shown, it will also show (to the right of the app icon) the name of any folder that it has been moved to.
You can also tap on the icon from within the search results to open the app.
Under iOS 10, you may have removed the app from your iPhone. If so, to add it back, open the App Store app, search for "Mail" and add the app back to your iPhone.