Been dealing with this a while now and I've had no luck searching for a solution so I'm just going to throw it out there and see if anyone has an idea.

Title says it all but here's the details. After several hours of up time, the desktop will crash. I have noticed it happening and can reduce the damage by dropping to shell (Ctl+Alt+F1/F2/etc..) and killing X which will restart the display manager so that I can log back in. In these cases, everything freezes but I still have keyboard and mouse input and the system still responds to keyboard input (hence why I can drop to shell). The clock freezes as well which I thought would be helpful because I can view the system logs at that time to diagnose the problem but nothing unusual seems to be happening in the logs.

If this condition is left untreated (killing X), the screens will eventually go black and the system won't respond to input forcing a hard reboot.

Note that I have disabled all the screen-locking/hibernation/screen-savers stuff as a first step toward diagnosing this issue but the problem persists.

Some details:
KDE Plasma Version: 5.11.5
KDE Framework Version: 5.40.0
Qt Version: 5.7.1
Kernel Version 4.9.76
OS: Linux 4.9.76, Gentoo, 64-bit
Using NVIDIA 390.25 on GeForce GT 710/PCIe

If anyone has any ideas at all on how to diagnose this issue, I would greatly appreciate it. I really love KDE and everything works great except this issue which is causing me to consider a different desktop and I really don't want to do that.

Bump!

I also face this issue.
Normally when I leave the PC on for a few hours, plasma effects will crash, leaving only the minimal resources to keep functioning, and the most annoying part is that i get screen tearing, doing everything that has any kind of scroll.
This has been happening since plasma 5.19 and using different distros (kde neon, kubuntu, pop_os, manjaro)
This is really annoying, and there only way for this to be back to normal is to reboot the system.

This is due to NVIDIA Graphics driver .
You can fix it by switching to Intel/Integrated graphics card (and only use NVIDIA, when you are doing something graphics intensive).