If the above answer didn't solved the problem, try this also :Running Disk Utility
• Insert the Mac OS X Install/restore disc that came with your Mac, then restart the computer while holding the C key.
• When you see the Spinning gear, then release the C key
• You should see the language screen which will lead you into the Installer environment
• Once on the Installer screen, from the first window and from the Installer Menu bar, select Utilities.
• Now choose Disk Utility from the Utilities menu.
• Click the First Aid tab.
• Select the hard drive icon displaying your Macintosh HD
• Click Repair. Disk Utility checks and repairs the disk.
• Once completed, quit Disk Utility
• Click on Installer Menu and reboot to your Mac DesktopReset PRAM and NVRAM
: Click Here
If everything failed then its better to do an Archive and Install of Mac OS, basically it means Reinstalling Mac OS. I think your OS files are corrupted and its recommended to reinstall Mac OS. Don't worry Archive and Install will not delete any of your files, settings, applications etc. Its just replace the OS files with new one.
Let me know the results before Accepting my answer....