Assuming you mean an iBook. Maybe not the best way to ask for help in a Mac thread, but anyway...
Have you got the system discs? If so, insert the one that's labelled "System Install Disc 1" and restart while holding the "C" key down. This should load the OS X installer, don't worry though, you're not going to proceed with installing OS X (not yet anyway).
At the OS X installer screen, click through the first screen where it asks for the language to use. Then go under the top menu and, depending on the version of the OS X installer, look under the "Utilities" menu and select Disk Utility (for 10.2 and earlier, you'll find Disk Utility under the "Installer" menu instead of "Utilities).
In Disk Utility, select the iBook's internal HD in the list (hopefully, it shows up?) and click "Verify Disk" button under the First Aid tab. If any errors are found, you'll need to click "Repair Disk" button to fix them. Assuming any errors are fixed, quit Disk Utility and Quit the installer, it should ask you if you want to restart - before doing that, it may be worth clicking the "Startup Disk" button and making sure the internal disk is set as the boot device. Then restart as normal.
If that doesn't work, or if you don't have the system disc, post back and we'll try to think of another way.