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 would respond with this as a comment but I don't have that option, assuming because my reputation hasn't yet reached 50.
To me this sounds like a couple of possibilities. I don't see mention of whether or not the screen gets backlight, only that "it doesn't turn on", so I'm writing this assuming that the computer chimes but the screen remains black. It's good that you've tried reseating the RAM however I would also try to isolate both DIMMs. Remove one of the RAM DIMMs, try powering on the Mac. Reinstall the removed DIMM and remove the other DIMM, try powering on the Mac. Try installing the DIMM from slot 1 into slot 2 and vice versa. See if there is any change in behavior after each of these changes. If the issue persists with each change then RAM is unlikely to be the culprit.
Assuming RAM isolation has no effect I would try resetting the PMU. You didn't mention the exact model of your PowerBook G4 but this Apple KB article should help you identify the correct process for your computer. PMU resets typically have results when the Mac simply isn't power on at all however I've seen them resolve "no video" issues as well in the past.
If both of these had no effect I would start looking at it as a hardware failure, with either the logic board or LCD/display assembly to blame. You did mention that an external display had no video as well so I would lean towards a logic board failure. I would find it odd that the failure occurred after reseating the RAM but it could be a coincidence.
Also, PowerPC based Macs (G3, G4, G5) typically will not boot from USB devices. There ARE some methods for getting some specific models to do so however they often come with mostly mixed results. Your best bet for installing 10.5 would be doing so from a firewire drive or from the install DVD.
Hope that helps! Let us know if you have any luck with bringing it back to life!
Best Answer
Yes, but it's not worth it.
The screen in that iBook is basically worthless: it's a small, not very good screen. You're better off spending any money on a slightly better display that–if nothing else–is larger.
However, if you really feel like it, you can do so.
First, you'll need to identify what display you have. Run the command
ioreg -lw0 | grep IODisplayEDID | sed "/[^<]*</s///" | xxd -p -r | strings -6. This will output two strings. Copy the first one which should look something like "LP133WX2-TLC1", then search it. This will show you what panel it has. Using that info, search for it's controller. Using the same example here's a VGA controller for that panel. This will allow you to use it as a monitor.Again, let me reiterate: this probably isn't worth it for that panel. You can get a larger, nicer panel for the same amount of money and time you'll invest. But if you feel like doing it, go ahead.