Sometimes when you're making changes within Open Firmware things don't always go right, and you end up bricking your computer. This is often evident when you boot up and then hear the boot chime, and then it restarts resulting in a boot loop. Otherwise it may not boot at all.

You can reset Open Firmware's default settings by performing the following, based on your computer:

Desktops

With your computer off, press and keep holding the power button. You hear the programmers tone (not the boot chime), but keep holding. Next you will hear a boot chime and then the system will enter Open Firmware with the default settings loaded.

Portables

With your computer off, press and keep holding power and the Command/Apple Key. You will hear the programmers tone, but keep holding. Next you will hear the boot chime and the system will enter Open Firmware with the default settings loaded.

Once your computer has booted into Open Firmware you should run the following commands to reset the NVRAM contents, and any other misconfigured settings:

reset-nvram

set-defaults

reset-all

Stuck with a black screen?

There was one instance where I had corrupted some settings in Open Firmware by running fcode-debug? = true instead of fcode-debug? true. This left me with a PowerBook that would turn on, chime, and only display a black screen. I tried the above methods which didn't appear to have any effect, and the key command for zapping the PRAM wouldn't work.

After plugging in an external monitor to the DVI port and restarting, it booted normally into Open Firmware and the display returned on the PowerBook. Now since I could access the Open Firmware prompt I reverted my mistake with fcode-debug? false.

Referenced and expanded from darthnVader's post on macos9lives.com