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Reboot.
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Check all cables.
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Investigate recent changes you made to your hardware or software.
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Run Disk Utility and repair your volume(s).
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Check the contents of your Trash for files you might have deleted accidentally.
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Check your Internet, wireless, and network connections to make sure they're still working.
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Run a virus scan, using your antivirus application.
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Disable your account's Login Items and reboot.
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Turn off your screen saver.
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Run System Information and check the status of your hardware.