You know you had messages! They're not in your Trash or Junk folders. Where did they go? Focus on the lower-left corner of the Mail app's view, and you'll see a ...
When you tap the icon, it changes from an open circle with blue lines to a blue circle with white lines. You can delete this filter by tapping it and tapping Delete Filter. Starting in iOS 16, you can pair filtering with a Focus mode. You can’t modify these criteria to add your own filters–you have to use smart folders on a Mac for that purpose. You look at your inbox on your iPhone and iPad and find it empty. That’s the Filter button, and you tapped it without realizing it.