If you haven't tried these suggestions already, you could try an SMC and PRAM reset:
If that doesn't help, try this: http://support.apple.com/kb/TS2570
If that doesn't help, try booting into the Recovery HD, select Disk Utility, select Macintosh HD and 'repair disk'. If anything gets changed click on repair disk gain until you get a clean pass. Restart as normal from the Apple menu. Command + r boots into Recovery. More info here: http://support.apple.com/kb/HT4718
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