Try looking here: http://support.apple.com/kb/HT1431#faq8

Resetting the PMU might do the trick. I know the ol' Titanium is sweet, but it also might be time to look at a newer model.

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...

And you can look at the repair guide here:

http://www.ifixit.com/Guide/Repair/Insta...

Have the same problem. Tried resetting and still nothing.

Try taking out your PRAM battery, shown in the remove PMU guide (it's the big thing marked 616-0114-A)

Disconnect the tri color lead from that to the PMU, start up the PowerBook (it should now work), then reconnect that cable and hopefully you should be left with a functioning PowerBook !