Boot from the iBook install CD's and see what the SMART status of your hard drive is, if it's verified then everything should be OK with it. Repair permissions and try to boot again, at worst you will probably have to do an archive and install.
If the drive is failing, hopefully you have access to another Mac, put the iBook in Target Disk Mode by pressing the T key on startup, plug it into another Mac and backup all your important files.
Next is to replace the hard drive if it is failing, this can be done by yourself if you are good with computers, or you can get your local Apple store or Apple repair shop to replace it for you although it may be a bit expensive.
Good luck,
Sean.