Please follow what @mayer suggested.
If in case you do not have the original system reinstall disks, you can download Ubuntu for PPC.
http://cdimage.ubuntu.com/lubuntu/releases/trusty/release/lubuntu-14.04.5-desktop-powerpc.iso
You will need another computer to actually burn the .iso to a DVD or CD. Then put this in the superdrive, force the powerbook to turn off, and then hold down "C" while turning it on again.
This will give you a working desktop environment so you can backup your files if your hard drive is still in a usable state. There should be a file explorer built in. Use an external USB drive to save your files.
If you are comfortable with the command line, you can also check the hard disk for errors.
https://www.fosslinux.com/1297/check-hard-disk-for-bad-sectors-and-errors-by-command-line-in-ubuntu-and-linux-mint.htm