When you execute Xconfigurator, the program may detect the wrong graphic card, so you will not be able anymore to load X interface.  It occurs even when at the initial installation the right graphic card has been selected.
 

Solution

In the console, type vi /etc/X11/XF86Config to edit by hand the configuration file.  Press the Insert key to switch in Insert mode.  Arrange the graphic card settings.  Type Esc to quit Insert mode, then type :w to save the file and :q to quit.

In the future you should edit again by hand the XF86Config file to avoid changing graphic card settings again.