I guess it’s worth confirming if it’s been a logout or a reboot as well. If you open a terminal and type “uptime” does the time match when you booted up or after you left it alone for a while?
Check the output of: dmesg -Tand have a look through: /var/log/messages
I would be focussing on errors, warnings and/or terms like “reboot, shutdown, logout, timeout, idle, etc.” to try to narrow it down what is happening and when.