Last week my parents' PC conked out the day before I was due to leave the country, so in the few hours I had to try and knock up a solution, I set up a very old second machine in the house running openSUSE 11.3 with XFCE. I'd previously replaced the default login display manager with GDM, which evidently has various bugs. Aside from the cursor continually spinning around showing as busy, when I added a new user account for my parents, the login box showed two entries under my own name. It's only on hovering the mouse over the latter entry that my parents' username shows up. Anyway, I had to explain this and a million other quirks to them in a rush, but the system was all working and they managed to get me an email from it the following day.

Since then, however, when they reach the login screen they say there is just an image of a computer monitor, with the OS version and domain, but no usernames to choose from or other options anywhere, and hence they can find no way to login. I have no remote access to the machine and can only talk over the phone. I suggested a few key combos to see if anything happened, and they booted once in failsafe mode, but no luck.

What could have happened? Is there a key that would show the users again? Perhaps there's a way to get to a console login, but could they get back to a GUI easily after that? Unfortunately my parents are really not adept at these things and every instruction I give has to be repeated endlessly after which they usually get it wrong a couple of times first, so trying to do anything administrative or more clever than a few key presses or mouse clicks is going to be nigh on impossible. Running a live CD would be likely out of the question since they'd need to change the BIOS boot order first.

Could they have accidentally switched console? And would doing so perpetuate the problem even after a hard reboot?

well sounds like you either have a xorg configuration problem or you are stopping in runlevel three. then you would have to login as root to type gdm.
also sounds like you did not set gdm as you default window manager.
tell them to login to the black screen as there user and then type startxfce4.
how to login at the prompt just type the user name and press enter type the password.

I think it's runlevel 5, since they sent me a little drawing in the post (sweet!) and it appears to be roughly the same GDM login screen but without any usernames listed or other options that can be clicked anywhere.

I did this in the sysconfig options in YaST.

That's the problem, I don't know how to get to a console login screen.

I wonder if they've clicked 'Suspend' or another option instead of shutting down previously (I found some old and vaguely related bug about this seeming to cause subsequent GDM login issues for other users).

Just had a long and excruciating call with the folks but have finally got them logged in again through a combination of trial, error and sheer luck. I'm worried that whatever caused the issue might happen again though, so perhaps somebody could throw some light on the following:

I got them to do a Ctrl-Alt-F1, login at a console and type 'startxfce4', which they say produced this output:

(the above 'display0' and 'X0-lock' might contain different characters to what I typed)

After an ineffective switch to Ctrl-Alt-F7 and back to F1 again, I then took the chance of having them log in as root and typing 'shutdown -r now', after which they got back to the GDM login with the usernames. So does that shed any light on why how they got locked out in the first place?

/tmp/.X0-lock and start again. that is a stale socket as root delet the user socket from /tmp. this happens when for some reason some one closed the xorg wrong and the /tmp/usrname-socket is left active. during reboot it is not deleted so you try to log inand the xorg thinks you are already loggd in.
very common when putting the GDM on your system the scripts did not delete the socket. So when you go to runlevel 4 that is login