It sounds as though you’re viewing a collection, perhaps search results, rather than the standard binder. If you view the submenu Navigate ▸ Collections ▸, you’ll see a checkmark indicating what’s currently open in the binder sidebar, and you’ll also be able to select “Binder” to return to the regular binder.
There are a few different indicators you can show or hide in the interface to help make it more obvious when you’re viewing a collection. The name of the collection will always appear at the top of the binder (whereas the standard binder won’t show anything there), but you can also enable View ▸ Show Collections to make the tabs of different collections visible at the top of the binder. This might just be “Binder” and “Search Results” if you haven’t created any other collections of your own. Clicking on the tabs will load them, just like going through the Navigate ▸ Collections ▸ submenu.