If your Mac can't boot up, don't panic. The built-in Recovery System can get you up and running
A flashing question mark or flashing globe on boot is one of the scariest sights imaginable to a Mac user. It means that the Mac is not finding the system software it needs to continue booting. You're not doing anything else on this Mac until this is fixed.
Since OS X 10.7 Lion, Apple has included the ability for Macs to restart using a special system called the Recovery System. Short of a hardware failure, the Mac's Recovery System can help get you back to working order. It'll let you reinstall OS X Mavericks over the Internet, provided your Mac can maintain a network connection.
Don't do this unless you're prepared to, at least for a short time, risk losing all the data on your hard drive. You can easily restore from a Time Machine backup, as well.
The first step is to establish that your Mac's hard drive (or in the case of many newer Macs, flash drive) is in good working order. The next step is to reinstall Mavericks.
To use Recovery System
To use Recovery System to reinstall Mavericks, you'll need:
- Your Mac.
- Your Apple ID and password.
- A working Wi-Fi or Ethernet connection to the Internet.
Check your Mac's hard drive
- Restart (or start up) your Mac while holding down the command and R keys.
- When the Apple logo appears, let go of those keys.
- The Mac will start Recovery System.
- A window called Mac OS X Utilities will appear.
- Click on Disk Utility then click the Continue button.
- Disk Utility will launch.
- Click the first disk icon that appears in the sidebar on the left.
- Click the Repair Disk button.
- If Disk Utility finds any faults it can correct, it will. If it fails, you should bring your Mac to an Apple Store an or Apple Authorized Service Provider for further assistance, or be prepared to replace the disk yourself if you're so inclined.
- Assuming Disk Utility makes any necessary repairs, select the Disk Utility menu then click on Quit Disk Utility.
If you have a Time Machine backup, now's the time to try it.
To reinstall OS X
- 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 after the first one (lawyers — geesh).
- 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 Mavericks 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 Mavericks 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.
If the recovery software runs into a problem that keeps it from working, you may be required to reformat or repartition the drive. That will wipe the drive completely. This is why I said at the outset that you need to be prepared for the loss of your Mac drive's data. At the risk of beating a dead horse, I'll emphasize again that having a backup is handy.
Hopefully it hasn't come to that, and you're off to the races.
I'm sure I've left some stuff out, so if you have any questions let me know in the comments.