Reset/initialize a Win10/8 "Modern" app? (Mail)

[Ichinisan]

Talking to someone that did the Windows 10 upgrade from Windows 8. The Mail app was working in Windows 8. Now, in Windows 10, it's just showing a plain screen with the Mail logo in the center. No folders. No accounts. No settings.

Is there a way to reset/initialize a "Modern" Win8 / Win10 app?