It was probebly because your monitor was configured incorrectly. Log in as root in the text mode, then open /etc/X11/XF86Config-4 in any editor (mcedit, joe, vim, emacs, pico) and search for 'Section "Monitor"'. Below you'll find lines beginning with 'HorizSync' and 'VertRefresh'. Open your monitor's manual and type the values you'll find inside (in technical details). Example:
(my settings for an old monitor)
Then save the file and run 'startx'. Graphucal mode should start.