Download and run this. Reboot after you run it. Does it find anything?https://www.malwarebytes.com/mac/
If that doesn't help or if doesn't find anything reboot the computer and right after you hear the startup chime hold down, Command-Option-P-R until you hear it restart again.
I that doesn't work we'll first try repairing the hard drive. Follow this link for instructions. you may want to print them out.https://support.apple.com/kb/PH22243
If it doesn't find any problems, reinstall the system, follow the link below. Reinstalling the system won't get rid of any files BTW, it will just install a fresh copy.
BTW it will have to download the full installer again which is a pretty big download, so all depending on your internet speed it may take quite a while to download then reinstall.https://support.apple.com/en-us/HT201314
After reinstalling go the Apple and the App Store, run all updates.