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What version of Mac OS X are you using ?
Lets run Disk Utility from Single User Mode. Turn your Mac off. Turn it back on and immediately hold down the Command (Apple) key and the S keys at the same time. Then type the following command when it boot into black screen with white letters...
/sbin/fsck -fy (note the space between fsck and -fy)
If you can't run /sbin/fsck -fy then type only fsk -fy
Hit return and the disk utility should run and let it finish. When it finishes type reboot and hit return. Let me know if it boot into normal mode.
Please let me know if that didn't work, so we can continue troubleshooting.