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Because of many issues, this type of problem may happen. So we need to try different things to fix the issue. First of all please try the following which is called as SMC reset.
Shut down the computer.
Unplug the computer's power cord.
Wait thirty seconds.
Attach the computer's power cord.
Wait ten seconds, then press the power button to turn on the computer.
If it not helps, please boot your system in safe mode. While performing safeboot mac will check and repair many issues itself. To start up into Safe Mode (to Safe Boot), do the following:
Be sure your Mac is shut down.
Press the power button.Immediately after you hear the startup tone
, hold the Shift key. The Shift key should be held as soon as possible after the startup tone, but not before the tone
Release the Shift key when you see the gray Apple icon and the progress indicator (looks like a spinning gear).
During startup in Mac OS X v10.4 through Mac OS X v10.6.8, you will see "Safe Boot" on the login window, which appears even if you normally log in automatically. During startup in Mac OS X v10.2 through v10.3.9, you will see "Safe Boot" on the Mac OS X startup screen.
To leave Safe Mode, restart the computer normally, without holding any keys during startup. Check the result then. If problem happening again, it may the result of spyware infection or a hardware problem with your screen.
So use ClamXav the free Antivirus solution for mac. Click here
to download clamxav. Install it on your system. Then run it and follow its onscreen instructions to perform a virus scan.
Let me know the result.