Ever since upgrading my PC hardware (graphics card) to Nvidia 970 and 1070 GTX I have been plagued with Ubuntu’s Display Manager constantly (and seemingly) randomly freezing. It was also occurring with Ubuntu 14.04 LTS.
The only solution I could find to combat this was to restart LightDM.

I would hit Ctrl + Alt + F1 to open up the shell and login. Then restart the lightdm service:

$ sudo service lightdm restart

This would restart the display manager and would stop the freezes occurring, for a while at least. The most annoying this was having to do this literally every time I booted or logged in.

After trying fresh installs, Unity tweaking and other ‘fixes‘ I finally made the transition to Debian 9 and the default Gnome 3 window manager. And to my delight it appears the problem has gone away!

I will be sticking with Debian from now on (at least until I upgrade hardware again).