Everything was just frozen, couldn't even change tty's. Had to reboot using physical power button.

I run aptitude upgrade daily, & haven't had to reboot for weeks, if not months previously. Yesterday I did reboot, & everything came back up as per usual, then today this.

The clock was at 01:09:xx, so I know what time it kicked the bucket. Which log(s), if any, can I look at to see what happened?

And then overnight the screen turned black with barely visible windows scattered across the borders with just their corners in the viewing range of the screen. In this case mouse & tty's worked & I was able to reboot from there.

Two possible causes:

1)
(Automatic ?) update of 'X Window System' coming
in from Ubuntu, while running 'X'. They updated this, lately.
But that's improbable.

2)
Maybe, a new cron-job running at 1 o'clock?

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À propos Your wrong reolution:

Possible Causes:

KSceen?
xrandr?
xserver?
xserver-graphics-driver?
graphics kernel-module?
changed DRM config? (Direct Rendering Management)

Wrong/changed file permissions?
Hardware Error?

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More details needed:
journalctl -b <bootnumber of freeze>
Last Entries written there.