Have you tried safe mode? Press left shift while booting. Also, Disk warrior may be of help. Is your machine set to boot up to your user desktop, or to login window?
Anyway, have a look at these options:
Q. My MAC will not boot._ What should I do?
A. You can "Safe Boot" OSX by holding down the Shift key immediatly after you hear the startup tone (not before). Release the key when you see the grey loading screen with the Apple logo. If you did it right you should see "Safe Boot" when the OSX loading dialog appears.
A. You can "Boot from CD" by holding down the “c” key immediatly after you hear the startup tone (not before). Release the key when you see the grey loading screen with the Apple logo.
There is a disk utility on the OSX installation CD which might fix the problem.
A. You can Boot OSX into “Single User Mode” by holding down the Apple and “S” keys immediatly after you hear the startup tone (not before).
You can then run fsck to repair the partition, following the on screen instructions.
A. Another mode you can try is the verbose mode._ Hold down Apple-V during start up) and let it go. It will return some errors (keys error, among others), but eventually it "might" get you to your desktop
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