You'll probably need to edit your xorg.conf . The file in question is /etc/X11/xorg.conf (assuming a standard configuration).
Now, what state does your computer boot to? If the text login prompt, just login as root or as a user with Sudo powers, then use a text editor(Nano is a pretty intuitive one) to open the file (as root).
If it boots to a deeply broken and unusable graphical environment of some kind try using Ctrl + Alt + F1(or other F keys) to reach a text terminal, then do the above.
Once you have xorg.conf open, you'll need to find a line: Section "Screen"
Below that, there will be one or more Subsection "Display" sections. You'll want to set the Modes value correctly for your screen. Mine, for example is "1400x1050" (Quotation marks are included in the original) Adjust this according to the size of your screen.
Also check that the settings in Section "Device" match what you know your graphics hardware to be.
Chapter 11.2 of the text below may also be of service.
http://www.novell.com/documentation/sus...fdoc/admin93-screen/admin93-screen.pdf
If that doesn't work, report back.
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