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That circle slash indicates that core system files are missing or they were moved to some place else. Not sure what exact process you quit in Activity monitor. You may need to reinstall the OS X. Before reinstalling, lets try some troubleshooting first. First attempt to your boot your Mac in to Safe Mode. Restart your Mac and immediately hold down Shift key on your keyboard for 15 seconds and this will boot into Safe Mode. Did that work ?
If not, then lets run Disk Utility from Single User Mode. Turn your Mac off. Turn it back on and immediately hold down the Command (Apple) key and the S keys at the same time. Then type the following command when it boot into black screen with white letters...
/sbin/fsck -fy (note the space between fsck and -fy)
If you can't run /sbin/fsck -fy then type only fsk -fy
Hit return and the disk utility should run and let it finish. When it finishes type reboot and hit return. This will restart your Mac.
Please let me know if that didn't work, so we can continue troubleshooting.
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.