Several times over the past few weeks I've had my PC (Ubuntu 16.10*) apparently lock up: desktop stops changing (clock stops updating) but the mouse (usually) continues to move. Sometimes I can SSH in, although more often I can get as far as entering username/password but never reach a terminal prompt.

Run 'top' in a console window on the screen beforehand.

I find Firefox can do this with some web sites that are too javascript-heavy, load averages up to 50 or more.