one day you wake up... and everything's gone!

[Karl Hegbloom]

I've had similar problems from time to time.  Usually, logging out and
back in fixes it.  You can kick out of X with Ctrl-Alt-Backspace if
there's not logout button.  You should not push the reset or power
button except as a last resort.

When logging out and back in does not fix it, try using 'Ctrl-Alt-F1' to
get to the Linux text console, then log in and 'rm .gnome2/session',
then log out, and push 'Alt-F7' to get back to the GDM login screen.
Log in now, and it usually starts working right.

I think that one of times that this kind of thing happens is when
something on the system changes out from under running software.  So,
it's right after an upgrade.  It can also happen when the system has
crashed or you've pushed the power button or Ctrl-Alt-Backspace in the
middle of a session.

-- 
Karl Hegbloom <hegbloom at pdx.edu>