Do you have a filter set? If the highlighted button in the screenshot is a red funnel then you have a filter set that may be limiting what you see. If so, click it and choose Clear at the top-right to clear your filter. If the funnel is grey as in my pic then you have no filter set and something else is going on. If no filter is set you can enable SQL spooling from the Database|Spool SQL menu, spool to screen. Set focus to the Object Palette and hit F5 to refresh. Run the queries you see in spool SQL in the Editor and see if you get results.
This happens to me any time I’ve got my window laid out for 1/2 (vertical split) of a 4K screen, then later open Toad on my 13" laptop screen. The object palette control can’t handle a change in resolution gracefully, so the split bar gets lost. If you can’t choose a table, you’ll never see any columns. The only thing I’ve been able to do is undock the palette, make the window as large as possible vertically, and hope I can adequately expose the bar so I can pull it down. After I pull it down I can re-dock the control and everything is back to normal.