On my laptop when it freezes on linux mint, I'm able to do control + alt + fn + f7, get the shell and login with my username. To go back to the desktop i do chvt 8 this gets me back to my current desktop.

On my laptop when it completely freezes (unresponsive mouse or keyboard) on mint 17.3, I'm able to do control + alt + fn + f7, then control + alt + fn+ f2. This gets to the shell, then I login with my username and password. To go back to the desktop I do chvt 8 this gets me back to my current desktop. To find out the active tty's type w then chvt to the current one.

That way you can resume your work without rebooting or losing anything.