Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
I'm sorry about the issue. The circle with slash indicates that the system files are moved, deleted or corrupted. We may need to reinstall the OS X. But first we can do some basic troubleshooting steps and see if it works or not.
First reset the PRAM and NVRAM of your Mac. Follow the steps below to reset the PRAM and NVRAM
Shut down your Mac
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.
If that didn't work, then restart your Mac and immediately hold down Shift key on your keyboard for 15 seconds and this will boot into Safe Mode.
Please let me know if that didn't work, so I will send you instructions to reinstall the OS X.
Remember, this is not my final answer. If my solutions didn't work please don't rate me bad or poor. Instead please reply back to me and I will continue to help you until we fix the problem or you are satisfied.