AppleJack

Despite what many people believe, even powerful machines such as Macs can brake down and their operating system can be rendered unusable. In situations like this, AppleJack can help you fix your system. It grants you access to a pre-boot environment, in Single User Mode, in which you can run diagnostics on the operating system and attempt to repair it. The application is free, it can be installed quickly and it is compatible with all popular Mac OS X versions.

Besides installing AppleJack on your Mac, there is nothing else you need to do, except for remembering the application if your system is rendered inoperable. In this particular situation, you have to go to Single User Mode (Command + S before the operating system starts up) and type in "applejack". This will present a series of options, on separate lines and you only need to type an option's corresponding number and hit the Return key.

The presented options allow you to repair your disks, repair permissions, clean up cache files, validate preference files and remove swap files. If you are not sure which is the best course of action, you can choose the "auto pilot" option, to run all tasks sequentially.

Besides typing "applejack" in Single User Mode, you can also type "applejack auto" to run the auto pilot option automatically. Alternatively, you can type "applejack auto restart", to run the auto pilot option and to restart the Mac automatically. In other words, if you type in this command, you won't even need to stay in front of your Mac, so you can return when everything is hopefully fixed.

If you don't want to take your Mac to the store to fix it or buy a new one, then AppleJack is the solution you need.