I was cleaning my keyboard with my computer suspended. When I turned on the monitor, my lock screen was totally different to what it should be like. I entered my password, and there was an error report for Ubuntu which I okayed. I hoped it was a one-off, but no!

Now when I lock my screen with super+L, my monitor turns off as though it is going into suspend, and when I move the mouse a black screen comes up before my desktop and the same strange lock screen. My computer suspends as you would expect, except that when I resume, I am faced with the strange lock screen.

I had a look to see if there was anything like it and came across a couple of examples. This seems to be the same problem, except when I login my lock screen is normal: What is this second login screen?. I tried the solutions suggested in that thread, but nothing works.

Same problem but I only have one monitor (no solutions to try in this thread): Lock screen is not working Ubuntu 16.04

When I lock the screen from terminal with dm-tool lock it is as expected, quickly going to the lock screen, and my traditional lock screen, until I enter my password, then instead of going straight to my desktop, it goes through the same thing as when I lock it with super+L — the long suspense and monitor shutting off, followed by the strange lock screen (when I enter the password there I am returned to desktop).

This is the strange lock screen (with my username redacted)

Traditional lock screen:

Please can anyone suggest what I can do to rectify this?!

Also, I know I only have myself to blame, but surely typing any combination of characters in the password field on the lock screen shouldn't be able to change a fundamental thing like this?!

It sounds/looks like I might have pressed the combination ctrl+alt+* which has disabled a lock screen or something. I haven't enough posts to post another link. Google "Xorg 1.11 Vulnerability: Bypass Screen Lockers With A Keyboard Shortcut" and it is the first post that looks/sounds like what I might have done, but my Xorg server is 1.18, so I haven't tried anything suggested in the linked thread there as it is four years old!