Reinstalling Trisquel (and all the packages you added on top of the default
system) would certainly solve the problem... but it would be more instructive
to fix it. LightDM is the responsible for displaying the screen where you
can graphically log in the system. So we already knew it fails to start.
Isn't there any additional information in the journal?
By the way, you can use the -p option of 'journalctl' to only red the message
whose "gravity level" is at least the number specified in argument (smaller
is more critical). For instance, to only read messages that are at least
errors:
$ journalctl -p 3
To get as well the warnings (but nothing more trivial such as simple
notices):
$ journalctl -p 4
