Welcome to Linux..
Here's a couple suggestions to get you started.
By default, when using a Knoppix Live CD, based on my experience, the sudo command won't work yet. You must use 'su' instead, to get yourself root priveleges. The live CD as I recall won't ask for a password because I think there isn't one by default. However, if It *does* then try to locate in the system menu somewhere the option to 'set root password'. I think that su will work though.
After you su yourself, you can do the same stuff that sudo would let you do, like using mc (midnight Commander -- type 'mc') to edit your config files.
I think you are on the right track that the problem is video-card or video-settings related.
When I installed my Slackware and a couple other distros over the last few months, the video config file that was created by default after installation caused my monitor to turn off!! Not very helpful.
The file you are looking for is located at /etc/X11/xorg.conf. You will want to check that there is a suitable video driver selected for your card/device and that the HorizSync and VertRefresh are appropriate for your monitor, and that the resolutions available are also able to be displayed by your monitor.
If you have the manual for your monitor, you should find the needed settings in it; if not, Google your monitor to get the specs.
Let us know if this gets you anywhere.
OH PS - you *might* need to use 'chroot' to get yourself into the ubuntu installation, if Knoppix is unable to edit/save files in the ubuntu partition. do like:
knoppix-root$: mount /dev/hda1 /media/somewhere (if the ubuntu partition is not mounted yet)
knoppix-root$: chroot /dev/hda1 (to CHange ROOT into the ubuntu partition)
<now find and edit and save the file>
And now test Ubuntu.
Let us know if this gets you anywhere
and Good luck!