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Restart your Mac and immediately hold down Command and R keys for 20 seconds and this will boot your Mac to the Mac OS X Utilities screen. From there, click Disk Utility. Then repair the hard disk from there.
or follow the steps to 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 how it goes.