Is that white square your cursor? Are you still able to move it around? If you hit ctrl-alt-f1, does the screen show a black tty? Try reinstalling drivers: http://www.nvidia.com/object/linux-display-amd64-260.19.12-driver.html
Is that white square your cursor? Are you still able to move it around? If you hit ctrl-alt-f1, does the screen show a black tty? Try reinstalling drivers: http://www.nvidia.com/object/linux-display-amd64-260.19.12-driver.html