Q When I ran Software Update on my iMac, connected to the Internet, listed updates included an EFI firmware update. When that update was running, I quit all open applications, ejected the modem icon, and clicked Shut Down on the EFI update window. I then pressed and held down the Start button as instructed, and saw the grey screen with an update progress bar. Unfortunately I then had to disconnect from the Internet, which caused the screen to go black. My iMac now refuses to start up, not even sounding the startup chime. What can I do?
A Disconnecting from the Internet is probably the event that precipitated your problem, causing a failed or incomplete firmware update.
You may be able to bring your Mac up by resetting its SMC and the non-volatile RAM (formerly PRAM). Disconnect all cables from your iMac including its mains lead, and leave it for at least 10 seconds. Then reconnect the mains lead whilst simultaneously pressing and holding the power button, to reset the System Management Controller (SMC). Release the power button, reconnect mouse and keyboard, then holding the Command, Option, P and R keys at the same time, press the power button to start up. Keep those keys held down until a second startup chime is heard, indicating that you have reset the NVRAM.
If your iMac will still not play, or do anything that looks like starting up, you will need to try a Firmware Restoration CD. Apple provides this in the form of a downloadable disk image, which you can then burn to CD with Disk Utility on another Mac. This process is detailed here.
If you cannot get it to load the CD, or it remains unresponsive, you will need an expert to work on your Mac: this is best through an appointment at your nearest Genius Bar, or with a good Apple engineer.
Updated from the original, which was first published in MacUser volume 28 issue 06, 2012.