Mac OSX booting to black screen

Boot to Single User mode - Hold "command (⌘) + S" while powering on the Mac

Check the disk
/sbin/fsck -fy

Mount the file system as writeable
/sbin/mount -uw /

Remove three files
rm /Library/Preferences/com.apple.loginwindow.plist 
rm /Library/Preferences/loginwindow.plist 
rm /private/var/db/.AppleUpgrade

reboot