The first option you have is to recover your system from an existing Time Machine backup. What you choose here depends on your specific situation.

There's four distinct options here for recovery, re-installation or support. The OS X Utilities screen, rather than your own desktop, signifies you've booted into your Mac's recovery partition. You'll know that you've successfully booted into your recovery partition when the OS X Utilities window appears, though it is normal if this takes a moment to appear. If the Mac boots up as normal, you'll have to repeat the process, taking care not to hold down the keys too late or release them too early. You can boot into the OS X recovery partition at any time by restarting your Mac (through Apple menu > Restart) and then simultaneously holding down Command R until the Apple logo appears. Internet Recovery uses an available Internet connection to boot your Mac from Apple's servers and run the same set of recovery tools as if you had access to them locally, including the downloading and re-installation of the operating system. If you can't get access to your recovery partition, which may be the case if your issues lie with your hard drive, machines that shipped with Lion or above (or those that have been updated with a patch that brings Internet Recovery compatibility) have another built-in service, Internet Recovery. It includes all the tools you need for restoring your Mac from a backup, finding help on the Internet, modifying drives and even reinstalling the entire OS. The recovery partition eliminates the need for physical restoration material (such as the traditional DVD most older Macs shipped with, or the USB drive that was available to purchase around the launch of Lion).

This partition, Recovery HD, lives in the shadows of your Mac's hard drive, invisible even to Disk Utility, and only ever comes to life when you need it. Your Mac has a built-in set of tools that form the Recovery System services which reside on a special partition of your primary hard drive that OS X set up during installation. In this tutorial, I'll show you through the various options for restoring or recovering your Mac and the process of bringing your Mac back to a clean, healthy state. There may be a time, in the life of your Mac, where you need to restore or recovery your machine, either to try and solve problems you've encountered or simply clean house and start from fresh.

How to open mac os x utilities