Here are the instructions for reinstalling OSX:
· Back up any files that you need to save.
· Make sure that you the laptop that you are planning to restore has an active connection to the internet.
· Restart the Mac in Recovery Mode (hold down the Command key and the R key during restart).
a. Immediately after you hear the startup sound, press and hold the Command and R keys on your keyboard.
b. Release the keys after the Apple logo appears.
· If prompted, select a Wi-Fi network, or connect your Mac to the Internet using an Ethernet cable.
· Use Disk Utility to erase the hard drive.
Click on Disk Utility > Continue.
Select the main volume and click Unmount then Erase.
Quit Disk Utility (Disk Utility > Quit Disk Utility).
Note: If erase is grayed out, you may not have successfully booted into Recovery Mode. Shut down again, and restart, holding down the Command and R key.
· When you have exited Disk Utility, click Reinstall OS X and Continue. Follow the instructions to reinstall Mac OS X.