I noticed that my waterfall was missing so I hit F5 to check if it was hidden but it didn't reappear. I assume I must have deleted it but when I go to place a new one, as soon as I let go of the mouse button after dragging it into the render window, a message pops up: "Object FXWaterFallThin2048x512 has been deleted"

I can place any of the others with no problems.