Powebook G4 12" freezes at startup phase

Dear all,

my Powebook G4 12" (model A1010) during its start-up phase remains stuck on the "initial gray screen" without displaying the apple logo. However, I can access the system through the console provided by Open Firmware.

I would like to ask you what could be the problem and where such problem may reside (hard drives, memory, main board, etc). Moreover, I would like to ask you how I could try to solve this problem.

Thank you in advance.