How can I fix permission issue when I cannot start Mac OS X?
Boot to single user mode by holding command + S right after powering the system on (before the chime &amp; apple logo). Once the system has booted to the command line, type these commands without the leading tick mark. Spaces matter, periods matter, it has to be exact.


/sbin/mount -uw /
cd /private/var/db
rm .AppleSetupDone
halt


The mac should shut off. Now power on - you will get played the normal welcome screen. Make a new user (try not to re-use one of the old names - "rescue account" might be a good choice). You can then go to the preference pane and see what happened to the users.