Hello ; Please check that you are not hiding the bottom layer from "Layers and Objects" palette, it is that eye who controls rather to display the layer or not so if you click on the eye the layer will disappear and the eye will disappear as well to leave its place empty so another click on the same empty place will make the layer appear again and the eye as well. :) 

Excellent! Thank you very much!

Also check that you have not hidden (the little eye) or disabled editing of (the tick mark) any objects: 

All good! Thank you for your reply. Yes, I had accidentally  clicked on the eye on the Layers and Objects palette.