El Capitan froze and rebooted (Macbook Pro Retina Mid 2012)
Run a sysdisgnose. That is how Apple receives feedback from seeding/beta customers. The keyboard shortcut is Cmd Opt Ctrl Shift Period. 

You can also run sudo sysdiagnose -f ~/Desktop/ in Terminal. The former will save to /var/tmp and the latter will save to the Desktop. 

Sysdiagnose will contain pretty much A to Z.

You should be sure to check the disks.txt, diskutil.txt, errorlog.txt, logs.txt, and the diagnostics folder.