Your app is probably just disabled.
To re-enable it :
1) Go the the settings
2) Scroll to "installed apps" in the APP SETTINGS section and click on it
3) In the apps list, scroll down to the Mail app you should see a red icone next to the app name that tell you it is disabled.
4) Click on the app and down the screen you should see three buttons, the middle one is the "enable" button.
5) Click on it and your app should be visible now on your phone.
Let me know if it helps