If the icon is still on your phone but not on the dock, lightly press and hold the icon until all of the apps start jiggling and then drag it back to the dock. Press the home button to stop the jiggling.
If the Mail app is no longer on your phone (and it isn't located via Search), you can use the instructions below from Remove built-in Apple apps from the Home screen on your iOS 10 device or Apple Watch - Apple Support
Restore a built-in app that you removed