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This usually happens if there is any issue with the system files or hard drive. We can try some basic troubleshooting steps and see if it works or not.
Solution 1 :
First reset the PRAM (it is a temporary memory) of your Mac. Follow the steps below to reset the PRAM and NVRAM
Shut down your Mac by holding down the Power button for 5-10 seconds.
Restart and immediately hold down Command-Option-P-R keys at the same time
Hold the keys down until the computer restarts and you hear the startup sound for two or three times.
Release the keys and let the Mac bootup.
Solution 2 :
If that didn't work, then turn off your Mac. Turn it back on and immediately hold down Shift key on your keyboard for 15 seconds and this will boot into Safe Mode. It will take up to 30 minutes to boot into Safe mode. If it is not booting into Safe Mode, the move to next solutions.
Solution 3 :
Turn off your Mac by holding down the Power button for 10 seconds to turn it off. Turn it back on and immediately hold down Command and R keys at the same time for 15 seconds and this will boot your Mac to the Mac OS X Utilities screen.
From there, click Disk Utility. Select your hard disk at left column (Macintosh HD or hard disk name). Then click Verify Disk (under First Aid). If asked to repair, click Repair Disk. Then click Repair Permission.
If you are using latest Mac OS X El Capitan, then you will not see Repair Disk option in Disk Utility. Instead click First Aid button and run it. Repair if asked. Restart from your Mac from Apple menu.
Solution 4 :
Restart your Mac and immediately hold down Command and R keys at the same time for 15 seconds and this will boot your Mac to the Mac OS X Utilities screen. From there, click Reinstall Mac OS X. Then follow the instructions to reinstall your Mac OS X.
(If that didn't work, then restart your Mac and immediately hold down Command, Option and R keys at the same time for 15 seconds and this will boot your Mac to installation screen. Follow the onscreen instructions to reinstall)
Please let me know if that didn't work, so we can continue troubleshooting. I will send you instructions in here, so you can take your time and do this at your convenience.