That’s going to be troublesome. See for example Nvidia drivers arent installing on pureos
So for starters you would need the nouveau driver to be recognised as needed, and working. Maybe someone can troubleshoot that but flying blind is always difficult.
Actual Librem laptops have Intel CPUs with Intel integrated graphics and that is probably the cleanest and best supported scenario for PureOS. (Actually works best with other Linux distros too!)
Your symptom suggests that it fails to initialise the graphics.
I guess it may help to specify what kind of display e.g. since you said “desktop”, I assume external display. Is it connected via HDMI? What native resolution? And what specific model of nVidia GPU?
Ah, if I had a dollar for every Linux boot that has done that … 
How long did you leave it?