My fingers fumbled over the keyboard, and managed to cause my Content Library to disappear.
How do I restore the icons and directories?
In the bar below the check boxes, just above the word None, there is a double arrow symbol - click that and it should show the folder list.
Oh, I see what I missed. Two of those little bars are stacked on top of one another... I'm afraid that this was one morning where everything was going wrong and I simply overlooked that.