When that happens, try Ctrl+Alt+F1. If that switches to a terminal session
(where you can log in), then the kernel is not panicking. From the terminal
session you can kill the problematic processes or even the whole graphical
session with 'sudo service gdm restart'.
To fix the actual problem, you should first understand it: read the logs (in
/var/log) of what happened just before a crash.
It may also be a hardware issue: I would check the RAM with memtest86+ (in
Trisquel's repository and to run from GRUB's menu).
