IBook G4 won't start

[Jay W]

Help. I have two white IBook g4's doing the same thing. They won't get past the grey apple and neither one will boot from a start up disk. I've tried all known user remedies including resetting the pram, removing the battery and power source, starting up with the c key pressed, the option key but nothing. I suspect the hard drive, but why won't they boot from a bootable disk?
Jay W.