Your Mac may start in single-user mode unexpectedly. When starting up in single-user mode, you will see a black screen with white text. The normal login window or desktop picture do not appear.
This can happen in Mountain Lion if you press the Power button on an Apple Wireless Keyboard at the same time as pressing the Power button on your Mac.
Tip: After the system starts in single-user mode, you can restart normally by typing "reboot" and pressing the Enter or Return key.
To prevent the system from starting in single-user mode, avoid powering on the Mac and the keyboard simultaneously. If the Mac is being used daily, it is not necessary to press the Power button on the keyboard when starting up or shutting down the computer. When you shut down the Mac, the keyboard that is paired with it will automatically enter a low-power mode, and it will automatically leave the low-power mode the next time the Mac is started up.
Only if the keyboard will not be used for a period of weeks, it is recommended that you turn off the keyboard and remove its batteries. If the keyboard has been powered off manually, you can prevent the system from starting up in single-user mode by pressing the keyboard's power button three seconds or longer before pressing the power button on the Mac, or you can press the keyboard's power button after the Apple logo appears while the Mac is starting up.