It’s hidden - the site owners/admins can still open, view and edit the page, but visitors can’t access it (even with the direct URL). To show it again, open Pages and in the page settings (cog icon) toggle on ‘enable page’.

It’s likely in the Not Linked area or tucked inside a folder. Open Pages, check Not Linked at the bottom (and expand any folders), then drag the page into Main Navigation to make it appear in your site header menu.