Hello,
First try resetting the PMU follow this link:
http://support.apple.com/kb/HT1431
If that doesn't work then your going to need your restore disk that came with your iBook or the Mac OS X 10.5 retail disk that you have installed on your iBook.
Boot from install disk. (Hold C while booting). Select Disk Utility from top bar and repair permissions and verify the disk. Reboot normally and see if it works.
If that fails to help you try:
"Repair" OS X. Boot from the Mac OS X 10.5 disk and reinstall OS X. This should NOT erase any data, just replaces the system files which may have caused your booting problem. Now install the latest combo update for OS X 10.5 and repair permissions on your drive with Disk Utility.
Best of luck.