iphone7 email icon is gone. how do i get it back?

First, swipe your finger down from the very top edge of your iPhone and then use the search field to search for "Mail". The app may be lost within a folder but a search would reveal if that is the case.

If it is no longer on the iPhone, then you can re-download it from the App Store. Open the App Store app, search for "Mail", scroll down to the only app called "Mail" by "Apple" and then touch "Get". If it says "Open" instead, then it's already on your phone somewhere.