My Ubuntu box restarted without warning, what log files do I need to examine?

My Ubuntu box restarted without warning, what log files do I need to examine? Specifically,, (1) which log files will tell me what forced a sudden restart? And (2) where do I find them?

Update: Following waltinator's answer I ran sudo journalctl -b -1 -e and spotted:

 smartd[1310]: Device: /dev/sda [SAT], SMART Usage Attribute: 194 Temperature_Celsius changed from 108 to 109

I suspect I have a thermal problem.

Also did this:

  [  me  ]:/var/log$ ls -al /var/crash
  total 4936
  drwxrwsrwt  2 root     whoopsie    4096 Jul 30 07:40 .
  drwxr-xr-x 16 root     root        4096 Jun  5  2017 ..
  -rw-r-----  1 [  me  ] whoopsie 4537678 Jul 24 12:50 _usr_bin_simple-scan.1000.crash
  -rw-r--r--  1 [  me  ] whoopsie       0 Jul 24 10:34 _usr_bin_simple-scan.1000.upload
  -rw-------  1 whoopsie whoopsie      37 Jul 24 10:34 _usr_bin_simple-scan.1000.uploaded
  -rw-r-----  1 root     whoopsie  498295 Jul 24 10:57 _usr_sbin_ippusbxd.0.crash
  -rw-r--r--  1 root     whoopsie       0 Jul 24 10:53 _usr_sbin_ippusbxd.0.upload
  -rw-------  1 whoopsie whoopsie      37 Jul 24 10:53 _usr_sbin_ippusbxd.0.uploaded

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.