Lost Mail icon or App on my iPhone. How do I restore it?

Hello,

Restore a built-in app that you removed

1. On your iOS device, go to the App Store.
2. Search for the app. Make sure that you use the exact name of the app. Find the correct name of built-in apps.
3. Tap to restore the app.
4. Wait for the app to restore, then open it from your Home screen.

If you have an Apple Watch, restoring an app to your iPhone also restores that app to your Apple Watch.

From:

Remove built-in Apple apps from the Home screen on your iOS 10 device or Apple Watch - Apple Support