I misread you. I thought you simply had a lower resolution than possible. So
just to be clear: you do not have anything graphical, right?
By Pressing Ctrl+Alt+F1, you will be presented a terminal session where you
can login (I believe the live system user is named "trisquel" and there is a
blank password). From there, you can use 'nano' (available in the live
system) to create and fill Xorg's configuration file:
$ sudo nano /etc/X11/xorg.conf
After saving and quitting (keyboard commands are written at the bottom of the
screen), you can try to launch the graphical login manager again:
$ sudo service gdm start
If it replies that it has already been started, write "restart" instead of
"start". You can try different configuration until succeeding.
You will probably want to take a look at Xorg's log to understand the issue:
$ less /var/log/Xorg.0.log
