This happened to me. Usually the file structure and disc need repairing. Here's what I do:
Startup from the OS X CD (use a pin to open the CD tray), wait for the OS X installer to boot. Don't run the Installer, instead go up to the Menu bar and select Disc Utilities and use that to verify and repair the hard drive, and repair permissions if need be (Jaguar only).
If the iBook was in OS 9 when this happened (the only time this happens to mine), I run Disk First Aid from the OS 9 CD first, then repair the Disk from the OS X CD. Then re-boot from the OS 9 CD, where I get enough of the desktop and control panels back to choose OS X as the Startup Disk, and restart normally.