So, here's a step-by-stop process for dealing with log in failures in OS X:
- Hold down the power button to force a shutdown, then start in single-user mode (hold down command-S until you see the "weird" black screen with Mach/Unix boot information scrolling down it)
- Repair the damage to your hard drive by typing:
/sbin/fsck -y [hit return]
- "Mount" your hard drive by typing
mount / [hit return]
- Delete your user's preferences folder. Do this by typing:
cd /Users/[yourname]/Library [hit return]
mv Preferences Preferences.old [hit return]
mkdir Preferences [hit return] - Reboot by typing:
reboot

