I just noticed on the top toolbar, I have an icon, when hovering over it, says “Software Renderer in Use. Rendering may be degraded”. See here:
I found this in an old thread: Black screen after login on recent update | accelerate software rendering - #7 by nepti
But when I edit the .config/kdeglobals file - those two lines are not there, so that can’t be it.
That said, when I clicked on the icon, it loaded a dialogue box, and I made sure that both drop-downs show “Automatic” (previously “Background renderer” was set to “software”) and hit apply, and rebooted. This restored everything. I have no idea what would have caused this to occur.