To repair a syteme damege of a computer
if its a Mac
Restart your Mac while holding down the Command and R keys.
When the Apple logo appears, let go of the keys.
The Mac will start Recovery System and a window called Mac OS X Utilities will appear.
Click on Reinstall OS X
Click the Continue button Another Continue button appears. Click it Click the Agree button to agree to Apple’s software license agreement Click the Agree button that appears make sure your Mac’s hard drive (disk icon) is selected and click Install
Type in your Apple ID and password then click the Sign In button.
The Mac will download a fresh copy of your operating system from the App Store and will install it. The speed of this process entirely depends on your Mac’s connection speed to the Internet. You can wait an hour or longer on a slower connection
Your Mac will restart automatically once the software has downloaded, then the installation of the operating system will continue.
If all has gone well, your Mac is able to restart and boot normally again. Under ordinary circumstances, your applications and data should be safe and intact.