So, I've been having this problem for a while, since 18.04 I guess. My computer reboots out of nowhere. It is really, really random. The only thing I've noticed is that it only happens when my CPUs are idle, it never happened while I was working on something or playing. Also, sometimes it doesn't reboot, but my screen goes entirely black and it never comes back, so I have to force the shutdown by pressing the button down. I've read many posts on that issue, but nothing worked. My system is:
-- Dell Inspiron 5675;
-- Kernel Version: 5.11.0-25-generic;
-- Graphics Platform: X11;
-- Processors: 16 × AMD Ryzen 7 1700X Eight-Core Processor (64 bit);
-- Memory: 7,7 GiB of RAM;
-- Graphics Processor: AMD Radeon ™ RX 470 Graphics
Also, I'm using KDE Plasma as the desktop environment:
-- KDE Plasma Version: 5.21.5;
-- KDE Frameworks Version: 5.80.0;
-- Qt Version: 5.15.2
What system log would be more helpful? I've run the command journalctl and found some ACPI error messages there. Anyways, thanks in advance. Any necessary information, I'll be more than happy to provide!
dmesg
is where I'd explore first if the system is still running; it only shows messages for the current boot; systemd logs (as you mentioned) for prior session(s) assuming you cleanly shutdown, but I'd try switching to terminal & use SysRq commands as per first comment as power-button tells you nothing; just looses info