Change into a tty, check your journalctl and see if there are any related errors for the current boot.

journalctl -b -p err

are you able to open terminal inside your KDE by any chance?

Sounds like issues with graphics drivers, can you revert to nouveau?

Do you have any possibility to log in as a different user or even create a new user to log in as? I wonder of some plasma config file could be corrupted.

I’ve had this happen on a newer Dell laptop and an Asus atx build. For both machines, a BIOS upgrade to the latest from their respective OEMs immediately restored functionality.

This happens to me once in a while with Fedora KDE. Usually right after a kernel update and I can resolve by going back to the previous kernel for a few days. Always guessed it was the AMD GPU driver.

Does KRunner (Alt + F2 default, sometimes Super + R in configs) still run? Try launching "plasmashell" from there. Or a terminal emulator so that you can see the logs of plasmashell, and maybe attach a debugger...