How can I fix permission issue when I cannot start Mac OS X?

Solution 1:

Boot to single user mode by holding command + S right after powering the system on (before the chime & 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.