It could be your DC-in board. The best way to test it would be to slide in another battery that has a charge to it... if the computer turns on, it is most likely your DC-in board. You can purchase one here if you are interested:
http://www.ifixit.com/PowerBook-Parts/G4-Titanium-DVI-DC-In-Board/IF133-010
And you can look at the repair guide here:
http://www.ifixit.com/Guide/Repair/Installing-PowerBook-G4-Titanium-DVI-DC-In-Board/94/1