I'm running 12.04 LTS, and as usual when the package manager prompts me to update I do it. I updated last night (Sept. 27) and now the system boots only into command line mode.
Before doing so I see the purple screen with the words "ubuntu 12.04" and the little dots going bright and dark, and then bam I'm in command line. I've tried using sudo apt-get to update my files, in case it was incomplete, and it tells me that everything is up to date. If I type the command "unity" I get a "fatal error" related to "no display variable set".
I have seen other questions and answers on this topic but I do not understand the answers provided. I am stuck. All I did was upgrade when the system told me to, what did I do wrong? I did not download any new video drivers lately.