Step by step:
1) change to a console with ctrl+alt+F1 through F6 (F7 is reserved for the X server)
2) Log in as root, type init 3 (you may need to press enter afte the init sequence finishes) then 'cd' to /etc/X11.
3) Edit the file XF86Config with vi (vi XF86Config) scroll using the arrow keys and find the line that reads: Section "Device". Change in that section the diver (to what ever it is) to 'vesa' (all lowercase). To do that press insert key (a message will appear in the bottom left corner) . When done, press escape and type :x (colon x) to save and exit.
4) Again at the command line type X. If a black screen with an X cursor shows up, press ctrl+alt+backspace to get to the console again. If the screen did not come up, pay attention to the error message on the screen.
5) If everything went OK, type init 5. Done!