No display signal after installing Debian on PowerMac 8600

[Lukas Foljanty <lukas@foljanty.de>]

Hi.

I am a total newbie to Linux, so perhaps this is all stupid, but I have 
a problem I cannot solve after installing the current build of Debian 
on my PowerMac 8600.

Here it goes:
I (successfully) installed the basic installation of Debian (CD 
distribution 1) for PowerMacs/PPC. I partitioned the hard drive, 
installed some modules and then finished the installation. When it told 
me that the installation was finished I restarted my Mac.

However, now I don't have a display signal! When booting of a split 
second, the display receives a signal, but then goes blank. I can hear 
the computer working and seemingly, it responds to keyboard inputs 
(from the sounds it makes), but since the screen is black, I don't know 
what's it's doing.

What's worse, I cannot boot in MacOS, either (neither booting from HD 
nor CD). So I am completely stuck here.

Help!

Suggestions are truly appreciated.

Thanks in advance!

Lukas