In the majority of cases, simply describing your issue will not provide enough information for someone to identify it’s cause and advise you on how to resolve it. It is important to include any system logs that may be relevant to your issue to ensure that neither your time nor the other person’s time is wasted. The following is a list of the most common system logs and where/how to obtain them. The logs are listed in the order of importance.

SYSTEMD JOURNAL
The systemd journal can only be accessed using the journalctl command. You can export your system’s journal from the last three boots using these commands:

su
journalctl -b -0 > /tmp/journal
journalctl -b -1 > /tmp/journal.last
journalctl -b -2 > /tmp/journal.2last
exit

XSESSION
Your session log is located in your /home directory:

~/.xsession-errors
~/.xsession-errors.old

XORG (X11)
You can find your Xorg logs here:

/var/log/Xorg.0.log
/var/log/Xorg.0.log.old

LIGHTDM
LightDM’s logs can be found here:

/var/log/lightdm/lightdm.log
/var/log/lightdm/x-0-greeter.log

POSTING LOGS TO THE FORUM
You cannot attach a single log file. Instead you must create an archive that holds all the logs you wish to include and then attach the archive to your post. Here is an example on how to create an archive of a folder named Logs that contains some logs to be attached to a post:

sudo chmod -R a+rw /tmp/Logs
tar -cf logs.tar /tmp/Logs