I had this issue. If you have only on screen, you can circumvent the screen by pressing Ctrl+Alt+F2 to send you to another terminal. There you can update your system or do something else. I received this bug and running sudo pacman -Syu on a second terminal kinda fixed it.
Though, right now (a day later) I have a bug where my laptop screen shows your screenshot while only my external monitor works.
Edit: This might not work actually. If I disconnect my external monitor, I end up in the same exact situation and updating my system did not work. It might have worked the first time since it was a fresh install and there was a lot to update.