Why doesn't the GUI display when using a DVI cable instead of a VGA cable?

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http://askubuntu.com – I was using a VGA cable on my desktop with Ubuntu 10.04. I switched the VGA cable for a DVI cable since the ATI card has sockets for both. Ever since then, the GUI will not display. I can get into the shell (text only) but I can not get a graphical display. (HowTos)