Navigator displaying blank

You've set the score to "Single page", set it to "Page View" or "Continuous view" instead
(Navigator is not blank, just very narrow)

Looks like you are using the special purpose "single page" mode, which is inherently incompatible with the Navigator since the purpose of the navigator is to show you the separate pages, but you've asked MuseScore not to show you those pages.

Normally, you wouldn't use single page mode, or the Navigator. Normally you would work in the standard page view, or in continuous view. And regardless of which view you are in, you can navigate simply using the standard tools like the standard PgUp/PgDn/Home/End keys, the mouse scroll wheel or equivalent touch gesture (with or without Shift to scroll horizontally), etc. But if you do wish to use the Navigator, just be sure not to use the special single page mode.