Start from your recovery system. OS X Lion or OS X Mountain Lion includes a built in set of utilities in the Recovery System. Restart your Mac and hold down the Command key and the R key (Command-R), and keep holding them until the Apple icon appears, indicating that your Mac is starting up. After the Recovery System is finished starting up, you should see a desktop with a OS X menu bar and a "Mac OS X Utilities" application window. Note: If you see a login window or your own desktop and icons, it is possible that you didn't hold Command-R early enough. Restart and try again.
When your computer finishes starting up, choose Disk Utility from the Utilities window.
Click the First Aid tab. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
Select your OS X volume.
Click Repair. Disk Utility checks and repairs the disk.