Hard drives are the computer components that break most frequently as they contain a disk that spins thousands of times per minute, and are relatively prone to breakage. If your computer was unable to boot (flashing question mark folder) then your first step is to determine whether or not your hard drive is physically damaged. Using Disk Utility to check your hard drive: Note: If your computer instead boots to the flashing question mark folder screen or a Mac login screen, then you did not hold down Command-R early enough or long enough, and you should turn off your Mac and retry. Once your computer has finished booting into OS X recovery, you should see the following screen: Continue holding Command-R until the Apple logo appears. Now locate both the Command and R keys on your keyboard, and start your Mac while holding them down. In order to access the recovery partition, power off your Mac. How to boot into the OS X recovery partition: The recovery partition is accessible even if your computer is able to boot, and can be used at any time to re-install Max OS X in the case of a virus or operating system corruption. In either case, your first step should be to boot into the OS X recovery partition in order to diagnose the problem. This usually means one of two things: either that your Macintosh Hard Drive has failed or that your installation of Mac OS X has become corrupted. Then your Mac is telling you that it is unable to find a bootable device, meaning that it is unable to launch Mac OS X. If, when your turn on your Mac, it displays this flashing-question-mark-folder-of-death:

The recovery partition is a collection of system recovery tools that enable users to diagnose and fix their Mac if it is unable to boot.

The OS X recovery partition is a tool that is built into all Macs running OS X 10.7 (Lion) or higher.

Macbook air boots to os x utilities