Typically, when you created a new page or post in WordPress, you saw a screen similar to this one.

However, after the recent WordPress update, your screen may now look like this.

As you can see, the black navigation bars along the top and the left have disappeared altogether.
Fortunately there is a simple explanation. And if you want to display the navigation bars again, there is a simple fix.
The explanation
The latest version of WordPress offers several ways to view your editing window. The standard view shows both the editing window and navigation bars, as in the first screenshot above.
WordPress also offers a view called Fullscreen mode, where the editing window appears by itself, as in the second screenshot above. The idea is that you might find it easier to view and edit your content without the distraction of the navigation bars.
The fix

To exit (or enter) Fullscreen mode, click on the three vertical dots that appear on the top right of the screen. The View submenu will appear.
Here, you can turn Fullscreen mode off (or on), as you wish.
While you’re here, feel free to try out the Top toolbar and Spotlight mode, to see if they make it easier for you to edit your content without distraction.
For those of you on my Peace of Mind Maintenance Plan, you might see this change the next time you log in.
As always, feel free to contact me with any questions you may have.