Ask Max: My Mac won't start up, I just get a grey screen with a restart prompt...
Last week I was running an install of the automatic software updates and was required to do a reset. The reset was taking forever so I shut the computer down and when I went to reboot I could not. The machine turns on, does the start up noise, gets to the grey screen with the apple but after 30 seconds a darker grey screen filters down with an error message, "you need to restart your computer. Hold down the power button for several seconds or press the restart button."
This does not work, I have tried holding shift to get to safe mode, this does not work either, any advice?
Your best option is to boot up off your OS X install disc and do an Archive and Install of the Mac OS. It will not overwrite your personal data, just the broken System files. Apple provides more information on Archive and Install as well.