You'll find the log files in
/var/log/, and if you've never taken a peek in there before, the number of files and subfolders present may seem daunting, as different processes generate different log files. However, once you start poking around in there and read a few, things should start making sense.
For example, on my main rig, I can read the
errors.log file with the
less command to reveal that the system can't load/save the monitor's screen brightness:
May 17 11:46:30 localhost [ 7.979480] systemd: Failed to start Load/Save Screen Backlight Brightness of backlight:acpi_video0.
May 17 12:07:54 localhost [ 8.328123] systemd: Failed to start Load/Save Screen Backlight Brightness of backlight:acpi_video0.
This is expected because the old monitor I'm using doesn't allow for brightness adjustments "over the wire."
The most common logs to pay attention to would be
kernel.log, but you may find others of use to you depending on your system.
Also, there are utilities like
dmesg -- a program that prints out messages from the kernel ring buffer -- which you can use to determine problems in your current session. Its output is usually color-coded so warnings, errors, and critical alerts are fairly easy to spot. Depending on what distro you're running, there's also the systemd
journalctl command which will let you scroll through an aggregate of your system messages.
journalctl -b will limit the output to everything from the last boot onward. Hope this helps.