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The instructions are different for different version of the Powerbook G4 (see https://support.apple.com/en-us/HT1431 The one that worked for me was to push Shift-Control-Option-Power on the keyboard, release for at least five seconds then push the power button. This worked even though the computer appeared dead except for a constant light on the latch button.
I had the same problem. through trial and error I discovered if I removed my battery and unplugged it, then plugged it back in (without the battery) it would turn on. Even now, the only way I can start it is no battery, pull the plug, insert the plug and it will start. Hope it works for you, too.