Ubuntu randomly reboots on new computer without logging any relevant events; how can I find out what's going on?
Solution 1:
After a "sudden shutdown", aka "system crash", and reboot, the terminal command sudo journalctl -b -1 -e will show you the end of the previous boot's log's. If there is no hint there, suspect power/ overheating.
Read man journalctl to see how to extract information from the logs.
You can find how I make use of journalctl easier at https://askubuntu.com/users/25618/waltinator.