What the heck happened to my iPhone app?

Ok I'm going to try and shorten this as best as possible. My 1 year old was playing with my phone and now one of my apps is missing. She can't delete it because it's programmed into the phone. It was one of the four apps that sits at the very bottom each page. Whenever I do a search for the Mail app I can find it but I can't replace the icon back to its place. How in the world do I put it back?

Plug it in

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