Your Mac may start in single-user mode if you press the Power button on an Apple Wireless Keyboard at the same time that you press the Power button on your Mac. When starting up in single-user mode, your Mac displays a black screen with white text and the normal login window or desktop picture don't appear.
If this happens, restart your Mac by typing "reboot" and then pressing the Enter or Return key.
To prevent this from happening, avoid powering on your Mac and keyboard simultaneously. If your Mac is being used daily, it's not necessary to press the Power button on the keyboard when starting up or shutting down your computer. Instead, when you shut down the Mac the keyboard automatically enters a low-power mode, and it automatically turns back on the next time you wake or start up your Mac.
If you don't plan to use your keyboard for a few weeks, turn off the keyboard and remove its batteries for travel or storage.