Plasma freezes after login

[René J.V. Bertin]

>the gear stops rotating, but the screen remains blank and is unresponsive.

Can you switch to a virtual terminal (Ctrl-Alt-F1 for instance) or log in via ssh?
(there are ssh apps for smartphones, in case you don't have another computer with them installed.)

I've seen cases of full lock-out except from SSH as a result of a GPU freeze - the last one of those I had with my i915-driven eGPU was when trying out an Electron-based app that tried something a bit too fancy. (IOW, a modern desktop with all bells and whistles could have that same effect, I fear.)

R.