El Capitan unable to boot
Boot to recovery HD and just reinstall the OS.


https://support.apple.com/en-us/HT201314


This is done by Holding the Command and r keys down while turning on the Mac. You will then be given the option for reinstalling Mac OS. This will not delete any of your files, merely replace just the damaged Mac OS files.

All permissions will be fixed on the system side - no apps get deleted, no user accounts or date is deleted.

Worst case, you'll need to set up a new admin user on the machine once it's reinstalled (boot to single user move and delete /var/db/.AppleSetupDone) to finish changing permissions on your user folder.

You could also boot to internet recovery if normal recovery fails.