No network loopback device after security update ubuntu 12.04
I have had similar problem.
My lo didn't start automatically at boot (after turning off IPv6 via sysctl and rebooting the server).
My first workaround was to start it manually with ifup --force lo. But after that I had to restart most of my services as they didn't work properly without loopback.
Finally this old forum post helped me to find out the proper and permanent solution:
sudo rm -rf /var/run/network/*sudo reboot
(Apparently there was some old .lock file there that prevented the loopback to start properly.)