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That screen indicates that the system files corrupted or deleted. It is possible that El capitan installation corrupted the OS X. Now you can reinstall your Mac OS X. Reinstalling OS X won't delete any of your files. It will just replace the corrupted system files with new one and that should fix the problem.
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.