How to Reinstall OS X on a Mac
Though we’d all rather everything works as intended with our Macs, occasionally something goes really haywire and OS X becomes either royally messed up or unusable. In these circumstances, sometimes the only solution to get things working again is to reinstall OS X system software (or, if you have a recent safe backup made, restoring from Time Machine is often valid as well).
We’ll cover how to reinstall Mac OS X system software only with Recovery Mode, this reinstalls the most recently available version of OS X that is (or was) actively running on the Mac. If performed correctly as described, applications and user data will be preserved and not be modified at all, since this approach only reinstalls the operating system and system files.