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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How to Troubleshoot Mac USB Issues (7 Steps)

Check that the USB device is connected to the appropriate jack. There is a USB jack in the back of most keyboards, but it often only works with a mouse.
Unplug any USB devices and wait a few moments before reconnecting them. You can often fix problems with your USB keyboard and mouse by using this method. Try restarting your computer if the malfunction persists after you reconnect the USB device.
Test your USB peripherals in another computer. If the problem exists in the second computer, you may need to have the device serviced or look for an updated version of the firmware that supports your USB item. Check the manufacturer's website for these updates.
Try different devices in the same USB port. For example, if your mouse if behaving strangely, see whether an external drive or an iPod is able to connect to the computer via that USB port.
Disconnect any USB hubs that you're using and connect each device one at a time to your computer. You need to troubleshoot the devices individually to find out whether the USB hub or the one of the devices is causing the problem. Be sure to test each peripheral with a working USB cable.
Reset the parameter random access memory (PRAM) and nonvolatile RAM (NVRAM) on your computer. Turn off your computer, then restart it and immediately press and hold the 'Command,' 'Option,' 'P' and 'R' keys on the keyboard. Release the keys after you hear the startup chime for the third time and your computer should boot normally.
Consider reinstalling the operating system after clearing your computer's hard drive. Be sure to back up all of your data before you erase the disc. You may also want to bring the computer to a Mac service center and have the logic board examined.
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