When I went to edit a post on one of my sites that I had not visited for a while, I saw the back-end code that powers Gutenberg rather than the visual editor.
After reading about the issue on the WordPress forums, I went to my user profile and at the top under Personal Options, the box next to ‘Disable the visual editor when writing’ was ticked. I unticked it and everything in the back end went back to the normal Gutenberg appearance.
So the solution was easy, but I do not recall checking that box in my profile – but who knows.
Just in case, I deactivated and deleted the Limit Logins plugin and installed and activated WordFence and ran a scan and the site is clean.