What exactly is the error message?
Do you get a console login prompt, i.e.
Ubuntu 7.10 McLaren tty1
It may be that you just need to rerun the X server config. Which X server are you using?
Under Debian-based systems, you would log in as root and run a command such as:
(changing 'xserver-xorg' for the appropriate X server package)
It should then take you through the steps to re-detect your video card.
If this doesn't work, please give details of the error encountered and, if you can, the contents of your xorg.conf (or equivalent).
Hope this is of some help.