KDE hosed - no display

[Anne Wilson]

This morning my F9 box gets to the login screen, then I get a white box on a 
black background.  The white box disappears and I'm left with a black screen.  
It seems that things carry on in the background, as, after a while, I get the 
chimes that signal that the system is ready to go.

I've checked that it isn't the kernel that's the problem - it happens on both 
kernels.  The session menu at login doesn't show any other dm, so I thought I 
must have forgotten to install an extra one.  I went into failsafe mode and 
first tried for updates, to be told that there are none.  I then tried to 
install gnome-desktop, figuring that it would pick up the necessaries from 
there.  It says that gnome-desktop is already installed.

At this point I'm at a loss to know what's next.  Advice?

Anne