Graphical environment died

I've been using a laptop with Ubuntu 12.04 LTS.

I recently installed a game, SpringRTS, and then installed a few packages needed to compile its source code. After installing those packages I was no longer able to start games anymore. I restarted the computer and now the graphical environment won't start at all.

###
The game is SpringRTS.
I installed it through Ubuntu Software Centre but, since it installed a very old version, I later followed these instructions:
http://springrts.com/wiki/Ubuntu_install#Ubuntu_10.10_and_newer
(add-apt-repository ppa:spring)
Game was running fine on both my GFX cards.

Later I followed these instructions to install the required development packages:
http://springrts.com/wiki/Building_Spring_on_Linux#Ubuntu
After installing those packages I was no longer able to start it anymore. The severe warning signs came when I thought of trying some other game (Super Hexagon under Steam) and it popped me up a Terminal saying it had to install something Mesa DRI. I managed to start Steam but not the game - it flickered the GUI but quickly came back to the desktop.

I restarted the computer and now the graphical environment won't start at all. The system is booting and I manage to get to a command line in Ctrl-Alt+F1 but Ctrl-Alt+F7 is stuck in a block of text and I don't even see anymore the initial screen where I can choose how to start Ubuntu (older versions, safe-mode, etc).
###
The computer is a laptop with an Intel GFX combined with a NVidia GTX 770M with Optimus. I had installed Bumblebee(?) and Primus to be able to use it. Games were playing fine with either of the cards.
###

I'm not figuring how to start debugging this... I'm sorry for possible lack of info and for not having tried more things but I'm so put down by this that it was a huge effort to even bring myself to come put this "ask" for help - I felt like just crawling into a hole and stay there contemplating how my computer had crumbled and the desperation of the idea of having to back-up/reinstall it.........