Rach, the folder with the question mark means the computer cannot find the startup system. This could happens because files that are needed to boot the computer are damaged, or the drive's directory is damaged. This can also happen when your computer has a dead drive.Try rebooting your computer while holding the Command R key. This will attempt to restart the computer using the Recovery partition. If it works, you'll know that your drive isn't dead. Use Disk Utility to repair the main partition. If it then download and install the operating system. If you can't reboot with the Command R key try using Command Option R which is an Internet recovery. If the drive's partition table were damaged but the drive itself is okay, you'll be able to use the Disk Utility program to create a new parition, install the OS, and then restore from your Backup. You can find plenty of information about this [https://discussions.apple.com/community/notebooks/macbook_air|on here] and here [http://forums.macrumors.com/threads/mba-mid-2012-not-booting-flashing-folder-instead.1615191/|as well]