Sounds like your hard drive may be failing. If you can, use target mode and check it with Disk Utilities or boot up from your original system installation disk or retail system disk, go to the second screen pull down menu to Utilities > Disk Utility, select your hard drive in the left hand column and see if you can repair the drive. Hopefully you have your data backed up.
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Your machine uses a 2.5" PATA hard drive, here's how to replace it: http://www.ifixit.com/Guide/iBook-G4-14-Inch-933-MHz-1-33-GHz-Hard-Drive-Replacement/189/1
Sounds like your hard drive may be failing. If you can, use target mode and check it with Disk Utilities or boot up from your original system installation disk or retail system disk, go to the second screen pull down menu to Utilities > Disk Utility, select your hard drive in the left hand column and see if you can repair the drive. Hopefully you have your data backed up.