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We can try some basic troubleshooting steps and see if it works or not. First try Disk Utility.
Restart in Safe mode Make sure that the computer is shut down. Press the power button. Immediately after you hear the startup tone, press and hold the Shift key. Release the Shift key when you see the gray Apple and progress indicator. Let the Mac boot up...
Let me know if that didn't worked, so we continue troubleshooting. Don't Rate my answer until you are satisfied.