Email icon missing from my iPhone screen

With iOS 14 it can be removed from the home screen without being deleted. If you have iOS 14, go to the App Library (swipe left past your last home screen). From there you can touch and hold an app and slide it left back to the home screen.

Another reason for it not being visible is due to a restriction. Go to Settings and tap Screen Time, tap Content & Privacy Restrictions, tap Allowed Apps. See: Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

If search won't find it ( Use Search on your iPhone, iPad, or iPod touch - Apple Support ), see the following from Delete built-in Apple apps from the Home screen on your iOS 14 or iPadOS device or Apple Watch - Apple Support

Restore a built-in app that you deleted

1. On your iOS or iPadOS 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.

Use the Spotlight Search option to try to find it, by swiping down from the middle of the screen.

If it's in a folder it will tell you which one.

You can also swipe to the very last Home Screen to access App Library and see if it's there. If it is, you can drag it back to dock by dragging it first to a Home Screen, and then to the Dock.

If it can't be found anywhere, it may have been actually deleted. In which case you can re-download it from the App Store here: link-> https://apps.apple.com/us/app/mail/id1108187098