I was even confused on this one. But here is the answer to get back to normal view.
There are some arrows that open and close the interface pane. See where my mouse is pointing…select the arrows to open and close the panes. Try this and see what you come up with. There are many of these little arrows that open and close stuff in the UI.
Sadly removing a content directory by accident is all to easy to do in the Content Library tab, one of the options in the right click menu is to remove all directories, so you could accidentally remove all the DS native and Poser runtime directories with a couple of mouse clicks.
Thankfully you can add them back the same way, but you do need to know where they are located on your HDD to be able to reselect them.