my ubuntu 16.04 server occassionaly reboots itself and i dont have a clue why? below is the syslog file content which have keyword named Shutdown
Nov 21 13:51:42 AB-active-server systemd[1]: Created slice User Slice of support.
Nov 21 13:51:42 AB-active-server systemd[1]: Starting User Manager for UID 1000...
Nov 21 13:51:42 AB-active-server systemd[1]: Started Session 1196 of user support.
Nov 21 13:51:42 AB-active-server systemd[18340]: Reached target Sockets.
Nov 21 13:51:42 AB-active-server systemd[18340]: Reached target Paths.
Nov 21 13:51:42 AB-active-server systemd[18340]: Reached target Timers.
Nov 21 13:51:42 AB-active-server systemd[18340]: Reached target Basic System.
Nov 21 13:51:42 AB-active-server systemd[18340]: Reached target Default.
Nov 21 13:51:42 AB-active-server systemd[18340]: Startup finished in 15ms.
Nov 21 13:51:42 AB-active-server systemd[1]: Started User Manager for UID 1000.
Nov 21 13:51:42 AB-active-server console-kit-daemon[21735]: (process:18348): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Nov 21 13:51:42 AB-active-server console-kit-daemon[21735]: missing action
Nov 21 13:51:44 AB-active-server systemd[1]: Stopping User Manager for UID 1000...
Nov 21 13:51:44 AB-active-server systemd[18340]: Stopped target Default.
Nov 21 13:51:44 AB-active-server systemd[18340]: Reached target Shutdown.
Nov 21 13:51:44 AB-active-server console-kit-daemon[21735]: (process:18386): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Nov 21 13:51:44 AB-active-server console-kit-daemon[21735]: missing action
Nov 21 13:51:44 AB-active-server console-kit-daemon[21735]: console-kit-daemon[21735]: GLib-CRITICAL: Source ID 739 was not found when attempting to remove it
Nov 21 13:51:44 AB-active-server systemd[18340]: Starting Exit the Session...
Nov 21 13:51:44 AB-active-server console-kit-daemon[21735]: GLib-CRITICAL: Source ID 739 was not found when attempting to remove it
Nov 21 13:51:44 AB-active-server systemd[18340]: Stopped target Basic System.
Nov 21 13:51:44 AB-active-server systemd[18340]: Stopped target Timers.
Nov 21 13:51:44 AB-active-server systemd[18340]: Stopped target Sockets.
Nov 21 13:51:44 AB-active-server systemd[18340]: Stopped target Paths.
Nov 21 13:51:44 AB-active-server systemd[18340]: Received SIGRTMIN+24 from PID 18387 (kill).
Nov 21 13:51:44 AB-active-server systemd[1]: Stopped User Manager for UID 1000.
Nov 21 13:51:44 AB-active-server systemd[1]: Removed slice User Slice of support.
the server's disk partitions have 60%+ storage free and inodes are used 1% only
Memory is 60G and intel Intel(R) Xeon(R) CPU E5-2620 v4 @ 2.10GHz
If I have an unexplained shutdown I go looking in my logs, all of them, they are likely the only chance you have to find out what happened. I look in the system log and find where it started to boot and then look at the earlier entries.
Sometimes there is no smoking gun, then you can ask your DC if they had a power event and to check their equipment. I recently has a server go offline unexpectedly a couple of times and it turned out to be a faulty rack PDU.