If your main account is an Admin level, then anyone messing about in there would
be able to change it, and do so most permanently. Unless the changes were only
done in the System Preference panels. Anyone could easily delete all user content
and also mess up the OS X easily, so it would not even restart once shut off...
With Leopard 10.5.8 or Tiger 10.4.11, the last two systems available that could be
running in survivor PowerPC G4 or G5 (non-Intel) hardware, these are similar to
each other and most handling of them would be fairly easy to understand.
There is a clock battery on the logic board that may need replacement, and that
could affect the function of the computer, to some extent even the booting of it.
A reset of the PRAM can help in some cases, if the battery that stores those
settings has gone dead. Some authorized Apple service providers may still work
on those and some of their techs may know how to troubleshoot or repair them.
If you have or could find a Missing Manual for Tiger (or Leopard) by David Pogue,
that may be helpful as a guide to either system. I've found these books at a small
fraction of their former retail price and have given several away to persons that I
had given an older Mac computer, to remind them of their Mac, just sitting around.
Second-hand or charity thrift stores can be a source for some older Mac hardware
accessories and quality books such as these manuals.
If you can take a screen shot or other capture then upload it via the camera icon in
the ASC discussion reply window, that can be helpful. So would more details in the
situation leading up to this change of appearance, lost files, or whatever's going on.
Been some years since I had a few eMacs. Then some G4 iMacs, & lost interest in
the reasons why I bought them in the first place, given hardware failure under Apple-
Care warranty, but nobody would fix them here. I gave one of my Macs many rides
to & from the main city here, most of the first year, and they never did fix it. Funny
thing was, I joined the ASC forums to ask questions & attempt to get support and
none of my original questions about those products survived. Nor do any existing
reports of the troubles I had with them, exist now either. Or in effect, say nothing.
So there are things you could recover from, and there are things that may have
gotten overwritten; if not just simple settings in the GUI or a missing user account.
In any event...
Good luck & happy computing! 🙂