This issue is not unfamiliar among PowerPC G4 users, in desktop or portable models.
The cause may vary and so when looking through incidents of a similar nature, try to
be patient and get past duplicate-sounding experiences as referenced by the sources.
There are more than a few in this search thread that sound like your situation:
https://www.google.com/?gws_rd=ssl#q=powerbook+g4+black+screen+on+startup
Depending on the PowerBook model, some have clock battery/capacitor on logic board
that once it fails, there can be startup issues as a result... So that's one to check. And
then there are the connections that may have come loose on the logic board, over years
of handling and small movement, these could finally have failed to contact finally. Or the
DC-in Board may have chosen to fail, so power may be unable to get past a circuit.
Troubleshooting based on a few symptoms can be... troublesome, tedious; or costly.
The iFixit.com guides have sections where people asked questions, such as:
https://www.ifixit.com/Answers/View/131619/PowerBook+g4+has+startup+chime+no+ima ge
The keyboard shortcuts to attempt to force a computer to start may be of no avail if
the hardware has defects, or if wires have come off their necessary junctions inside.
If you have time and some money, companies who specialize in repairing Apple
portable computers could restore its function & warranty their work. wegenermedia
is worth contacting if the computer is one you want to keep around while. They did
a repair for me on one of my Apple portables, and I'd probably go that route again.
So, anyway, if the unit uses a clock or PRAM battery, it may need replacement.
Or in another direction, issue could be a power related circuit failure; or worse.
There are graphic display components that fail at a point where an external &
internal display both won't light up. These are mapped out in iFixit guides or in
others online, depending on the model G4 PowerBook they show.
Good luck & happy computing! 🙂
