Restart in Recovery Mode and from the menu options, choose Reinstall OS X and click Continue.

Start-up in Recovery Mode by Restating or Powering Up and holding down both the Command (⌘) + R key combination immediately upon hearing the startup chime. Select Disk Utility and check your hard drive for errors.

Your computer will restart when you hear the second startup sound release the keys. Reset NVRAM: To do that, turn off your Mac, then turn on, when you hear the start-up sound, press and hold the Command-Option-P-R keys.This article explains how to free up space on your Mac. Did you see any low disk space warnings before this problem? Your Mac should have at least 9 GB of free space. If your Mac can start up, the problem may be that your computer does not have enough space. To do this, turn on your Mac, when you hear the start-up sound, press and hold the Shift key until you see the Apple logo. I previously wrote an article about how to fix Mac startup problems.Wait a few seconds and then press the power button to turn on your computer.

Press and hold the power button for a few seconds until your computer turns off, then release the power button.

Fix Mac OS X El Capitan Perform the steps below one-by-one until your problem resolves

Mac pro 2007 el capitan