-sh-2.05b# What does this mean?

For some strange unknown reason your '12 PowerBook G4 has booted into Single User Mode. Usually when this happens you invoke it by holding down Apple/Command+S immediately after your Mac bongs. The fact that yours did it without being told to do so indicates your file system might be damaged.

You need to run mount -uw / by typing the command into the shell prompt then typing enter/return to tell your Mac to execute the command. After that you'll need to run fsck -fy until fsck reports that your hard drive is OK. Like before with the mount command you need to type return to tell the shell to execute the command.

Here's an Apple Knowledge Base Article with detailed instructions: http://support.apple.com/kb/TS1417. Normally I'd tell you to run Repair Disk & Repair Permissions from your Mac OS X Install CD/DVD but your Mac would boot into the GUI instead of Single User Mode if it were running normally. Because your Mac automatically booted into Single User Mode it would be wise to repair your file system using fsck.

Dave:

Invalid sibling link

Check out the article A possible fix for invalid sibling links on a hard drive.

😉 cornelius

Dave

unfortunately I don't have the disk to boot off.

That is unfortunate, indeed. If I may, I suggest that you get a disk before long. Apart from the fact that it can be very handy to have one in situations like the one in which you now find yourself, it can be used for regular maintenance, and for re-installation, if necessary. Besides, without having the install disk for the OS currently installed on your computer, you find yourself in a gray area, as you risk being in violation of Apple's Software License Agreement.

Good luck at the Genius Bar.

😉 cornelius