I had a problem just like that with my G3 500 PB.  They had to replace the
cpu.  My problem occurred when I was using System Disk to change the boot
behavior.  System Disk failed to write the eeprom and there was no next
reboot.

peter

>>>>>> "russm" == russm  <russm@webtime.com.au> writes:
> 
>>> At the moment my tibook is completely 'broken' because it no
>>> longer activates the display after booting!? Even
>>> Command-Option-P-R doesn't change this misbehavior. (Also an
>>> 4.1.8 firmware upgrade effect?) Anything I can do here?
> 
>   russm> Is this even when trying to boot MacOS? From a CD? Even the
>   russm> hardware test CD?
> 
> Well, I can't see anything. The tft display remains black all the
> time. There's also still a CD in the drive which I cannot eject... :-(
> 
>   russm> This could be your out - if the machine seems dead take it
>   russm> back for service.
> 
> Looks like...
> 
>   russm> Along that line, the install docs mention that setting
>   russm> load-base (or is it real-base) on OF3 machines will ruin
>   russm> them requiring a return to Apple.
> 
> ... and Command-Option-P-R cannot restore these values!?
> 
> Markus.
>