Navigation bar is on live site but nothing under menu so can’t add in a page

It’s possible that the developer hard-coded the menu items in the theme. Check out the Appearance, Customize screen to see if there is a Menu choice in there somewhere.

It may also be that the active theme (assume it was by the developer) doesn’t support Menus. That would be a warning sign to me that perhaps the theme is doing other non-standard theme stuff. You may need to find another theme, and use it. If you are not comfortable doing that yourself, you may need to find another (better) developer to help you out. But I’d check the new person’s references first to make sure that they are doing things properly.