I have 12.04 LTS, I don't think I installed anything extraordinary lately, (just uninstalled lxc-docker), but after last automatic update I am left with console. I can do

service lightdm start

and everything works fine. I checked upstart jobs, and lightdm is not there until I start it manually. I have followed http://upstart.ubuntu.com/wiki/Debugging and added verbose mode, but no information about lightdm. I also reconfigured lightdm, no change. I checked /var/logs but did not find anything useful in syslog, dmesg..
My question is, how to debug the problem?