Mac crashed after changing disk permission
If you just selected Macintosh HD and changed "everyone" to "No Access" and did not select "Apply to enclosed items", try to:


Start up in single user mode by holding command-S on startup.
Run mount -uw /. fsck -fy is not necessarily needed.
Run chmod 755 /.
Run reboot.


If that doesn't work, try to reinstall OS X from the recovery partition. It installs a new copy of OS X over the current installation while keeping user files and settings in place. It solved a problem I had where my Mac got stuck during the startup process. If nothing else works, try to erase and reinstall OS X, or erase the Macintosh HD partition (not the whole drive) from Disk Utility and then choose Reinstall OS X.

If you don't already have backups, try to save a disk image to an an external drive in Disk Utility on the recovery partition or copy files to an external drive in single user mode.