Do you have access to keyboard in OpenFirmware-console or in MacOS?
If it does, then it's MOS related. In this case your posidon-prefs may be corrupt. So try to boot without startup-sequence (hope you can reach early startup-menue by pressing F1 or left mouse-button during booting).
Now you in a shell and should have access to your keyboard and can rename posidon.prefs:

cd prefs/env-archive/sys
rename posidon-prefs as posidon-prefs.old

Now reboot! New posidon-prefs will be generated during run-time.
Hope that will help...