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.