My system is an AMD64 with Dapper Drake flight 3 and all the latest updates. After using the system for a while this evening, I rebooted. It would only boot into text mode, and displayed this problem:
GDM: Xserver not found: /usr/X11R6/bin/X :0 -br -audit 0 -auth
/var/lib/gdm:0.Xauth -nolisten tcp vt7
Error: Command could not be executed!
Please install the X server or correct GDM configuration and restart GDM.
Next screen:
The X server is now disabled. Restart GDM when it is configured correctly.
From the shell, I found that:
1. /var/lib/gdm directory was empty
2. /usr/X11R6/bin/X -> ../../bin/X which did not exist
3. When I ran /usr/bin/startx, it said:
Xauth: creating new authority file /root/.server auth.5268
Xauth: creating new authority file /root/.Xauthority
Xauth: creating new authority file /root/.Xauthority
/etc/X11/xinit/xservers: line 2: /usr/bin/X11/X: No such file or directory
/etc/X11/xinit/xservers: line 2: exec /usr/bin/X11/X: cannot execute. No such file or directory.
giving up [note: that's what it said, not me ;-)]
xinit: Connection refused (errno 111): unable to connect to X server
xinit: No such process (errno 3): Server error.
Note also that:
1. I wrote that all down by hand so maybe not every digit is 100% correct.
2. I presume this means that X was somehow wiped out during apt upgrades since I wasn't doing anything manually with X when this happened.
3. I am not sure if libx11 is the applicable package name for this sort of problem.
Don't know if it matters but I have this model of integrated graphics controller:
VGA compatible controller: ATI Technologies Inc RS480 [Radeon Xpress 200G Series]
The good news is that I did an "apt-get --reinstall upgrade" which fixed the problem. It reinstalled a dozen or so files, among them x-window-system-core, x11-common, and xserver-xorg. Then a quick reboot and I was "back in brown" once again. :-)
I have seen others with similar comments on the Ubuntu mailing list, that they did an update and ended up booting to a shell prompt rather than to Gnome. One of them suggested to symlink /usr/X11R6/bin/X to /usr/bin/Xorg, which they said worked for them. I believe the command they are referring to is "ln -s /usr/bin/Xorg /usr/X11R6/bin/X". Not sure if this would have worked in my case, but it might have.