Hi Peter, this is the potted version:
See if you can start the Powerbook
1. with Safe Startup (press shift at the chime and hold until the cog appears or a progress bar) This startup is slow, be patient. Safe start will avoid bad kernel extensions (.kexts) and a variety of other gremlins leaving a basic OSX running the Mac. It does clear up some problems as well.
If this works you may need to clean system and other caches using Onyx (free, download the correct version from VersionTracker for your version of OSX).
2. as a Firewire Target disc drive using a firewire cable to another Mac (press t at startup chime, the display will show the Firewire symbol like a screen saver. Attach to a second Mac, and the powerbook will show up as an external drive).
If this works you can rescue your data and reinstall OSX
3. after resetting management setups (see below).
start with PRAM/NVRAM Read Apple instructions carefully and be sure you follow them, particularly the SMC reset button push.
Info resources (Apple Support documents)
Mac OS X keyboard shortcuts
Last Modified: January 18, 2010 Old Article: 75459 Article: HT1343
Intel-based Macs: Resetting the System Management Controller (SMC)
Last Modified: December 09, 2009 Article: HT3964
Resetting PowerBook and iBook Power Management Unit (PMU)
Last Modified: January 15, 2010 Old Article: 14449 Article: HT1431
Resetting your Mac's PRAM and NVRAM
Last Modified: January 22, 2009 Old Article: 2238 Article: HT1379
When to reset NVRAM or PRAM
Last Modified: May 22, 2009 Article: HT1895