From what you're seeing I'd guess that the startup drive on your Mac has some sort of file system or directory problem.
If this were my computer, I'd reformat the hard drive and (assuming that went without a hitch) restore from my backup.
If a backup wasn't available, I'd run DiskWarrior (be certain it's a version compatible with the OS on the "borked" drive.) If that failed, I'd try the repair disk function in Disk Utility as a last resort.
Note that it's quite possibile that Disk Utility will repair this disk. Given that Disk Utility is free, one might wonder why I'd opt for the utility that costs money. My reasoning is that DiskWarrior is non-destructive and allows you to preview the "repaired" drive before committing to changes. If you absolutely can't or won't use DiskWarrior, at least use Disk Utilities "verify" mode before attempting the repair.
Unix Gurus might advocate using fsck instead of Disk Utility but I prefer the GUI approach.
I would also advocate casting a suspicious eye on any disk that you've had to use drastic methods to recover. Given the age of the computer in question, and assuming an original disk drive, my tendency would be to immediately transfer recovered information to a new disk and install that into the computer. In the old days I might put the old disk to work as a "shuttle" drive but in this era of keychain jump drives and DropBox I'd just strip it for bits or send it to the ewaste recycler.
I am pretty sure the Powerbooks don't boot from USB drives. You'll need to burn a CD.
I'm going to assume that you downloaded Ubuntu from the homepage at Ubuntu.com, which will give you ISOs for Intel (x86 and x64) processors. Since your computer has a PowerPC processor, you will need to download that ISO: Ubuntu's wiki for installing on PowerPC
Edit - Someone on the Ubuntu forums says you should be sure to download the livecd version and set arguments for loading the kernel:
Be sure to down load the powerpc live disk. 8.04.1 will give you the least trouble with booting. On my Ibook I have to press the tab key as the first words come up as it begins to boot. This takes you to a screen with some options to load the kernel. The one that works for me is "linux-nosplash-powerpc".
Best Answer
If you have another Mac nearby with a FireWire port, you can likely book the PowerBook in Target Disk Mode. Power it down, then hold the T button when you turn it back on until a FireWire logo appears on screen. You can mount it as a drive on another Mac, and restore the files you altered from your backup.
If you don't have a backup, you could at least use Target Disk Mode to make a backup of your data so you won't lose it when you restore the PowerBook from its restore CD/DVD.