So I was upgrading my home server first from Maverick Meerkat to Natty Narwhal, and then from Natty to Oneric Ocelot.
It is not a plain desktop installation, as back in the time I installed Ubuntu Server and then built upon it adding Gnome without the useless stuff that comes with the ubuntu-desktop package.

Anyway, after upgrading to Oneiric the X interface went away, all I saw was the boot messages text by the kernel up to the Apache2 start, and nothing else. SSH was still accessible so I could go through it, but you could still use recovery console to access the system if you don’t have remote terminal capability installed.

Checking with dmesg I saw these error messages:

[ 24.974182] gdm-simple-slav[1009]: segfault at 0 ip 002945b7 sp bfe9b6c8 error 4 in libnss_compat-2.13.so[291000+6000] [ 38.598946] gdm-simple-slav[1218]: segfault at 0 ip 00a3b5b7 sp bf9c35c8 error 4 in libnss_compat-2.13.so[a38000+6000] [ 39.562834] gdm-simple-slav[1238]: segfault at 0 ip 005eb5b7 sp bff72138 error 4 in libnss_compat-2.13.so[5e8000+6000]

Upgrading again, via SSH, to Precise Pangolin didn’t solve the problem, so I googled aroung and found this bug on launchpad.

Apparently, the autologin feature prevents GDM from going on and just hangs there.

Briefly, what I did and worked in my case (mileage may vary) was:

sudo add-apt-repository ppa:gnome3-team/gnome3
sudo add-apt-repository ppa:ubuntugnometeam/ppa-gen
sudo apt-get update
sudo apt-get dist-upgrade
sudo mv /etc/gdm/custom.conf /etc/gdm/custom.conf.off

The last line is the command that removes the autologin (by renaming the conf file that activates it), after doing this and rebooting I was showed the login screen.