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I started to have this issue without doing any updates or upgrades. My laptop suddenly became unresponsive and I had to do a hard reset. The computer boots up and the first suspicious thing is that I see the plymouth purple screen, but inside there is a big black rectangle. Afterwards I log in as usual, but only a black screen appears. I can see my mouse cursor, but I cannot interact with the system in any way. Also, a strange thing, the system fan starts going full speed a couple of seconds after I login. This would indicate that the system is doing something.
I tried to reinstall the desktop completely using this guide. No effect.
I tried to uninstall Nvidia drivers following this answer. No effect.
My issue seems to be related to this bug, but I am not sure, since it is not clear whether or not the problem described therein remains after reboot. Mine does.
I tried to test out all the linux kernels that I have available in my GRUB's advanced boot settings for Ubuntu, again to no avail.
I am truly perplexed, since I am innocent of any tinkering with drivers or updates. Any guide whatsoever is most welcome.