CentOS - Dec 2016 - GUI black screen on login, startx works???

Hello,
I'm using a recent install of centos 7, no GUI customizations that I
recall. When I log in using the GUI it accepts my password and the mouse
appears on a black screen, like it is preparing the desktop, but the
desktop never appears. If I hit control-alt-f4 and log in using the command
line, and then execute startx, Everything seems to work perfectly.

What sort of troubleshooting should I do to figure out what is stalling?
I'm not at all familiar with this aspect of things.

thanks,
Dave