KDE will not start (blackscreen)

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Table of Contents
  1. Is X working?
  2. sddm
  3. nvidia drivers

When KDE won't start, it may be due to a huge variety of reasons.
Use this page to document further the debugging workflow that will lead to the proper solution for each case.

Is X working?

Signs that X Server is working: You can see a black screen, and maybe a mouse cursor shaped like an X, but nothing else.
In tty1, you can do:

# ps aux | grep tty7

and see that X.org is running.
If you kill the X.org process, it restarts automatically, with the same results.
Thus, X.org is properly running and is not the culprit.

Check /var/log/

sddm

Case:
In /var/log/system, you see something like:

Dec 01 17:06:16 user sddm-greeter[576]: Failed to create OpenGL context for format QSurfaceFormat(version 2.0, options QFlags(), depthBufferSize 24, redBufferSize -1, greenBufferSize -1, blueBufferSize -1, alphaBufferSize -1, stencilBufferSize 8, samples -1, swapBehavior 2, swapInterval 1, profile  0)

This hints at a video driver problem.

nvidia drivers

Check if there is a problem with the nvidia drivers, or that your card is supported by the current drivers.

On a ubuntu system, check if you have any nvidia drivers installed:

dpkg -l | grep nvidia

Try removing them, so that the system defaults to using the open source nouveau drivers, then reboot:

apt-get remove --purge nvidia-304
apt-get autoremove
shutdown -r now