How to Restart From Apple Grey Screen

Attempt a basic restart. Press the 'Command,' 'Control' and 'Power' buttons simultaneously. Your computer should automatically restart unless completely frozen.
Hold down the 'Power' button for about 10 seconds to shut down the computer. Then press the 'Power' button again to start up the machine.
Disconnect any external devices and peripherals such as printers, scanners, external CD/DVD drives and other devices. Remove any Ethernet cables as well, and try restarting again. Sometimes a faulty peripheral will cause your startup to fail.
Shut down your computer again if you continue to see a gray screen on startup. This time, start up your computer again and immediately hold down the 'Shift' key to perform a safe boot. If your computer starts up successfully, try restarting to see if the safe boot resolved your problem.
Reset your paramater RAM and nonvolatile RAM. Shut down your computer, start it up again and immediately press and hold the 'Command,' 'Option,' 'P' and 'R' keys simultaneously. Make sure to press these keys before the gray screen appears. Release the keys when your computer starts up successfully.
Insert your Mac OS X installation disc into your CD/DVD drive and restart your computer. As soon as your computer screen begins the startup process, hold down the 'C' key on your keyboard to use your installation disc as a startup disc. When your computer starts up, select your 'Disk Utility' option and perform a basic repair. Restart your computer again.
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