It sounds like the display has been set to a value beyond its native resolution. If that's the case, then you probably have a couple options.
1) If there's a safe-mode you can boot into (say when GRUB loads), try and use that and then change the resolution from there.
2) Use the Ubuntu disk as a rescue disk (on the assumption this is possible---not being an Ubuntu user I don't know what tools you have immediately on hand).
3) You could reinstall Ubuntu from scratch.
4) And if you're feeling adventurous, boot into the command prompt and edit /etc/X11/xorg.conf, removing the offending display settings and replacing them with lower values. (The section you'd be looking for is "Screen". The display modes will be below, one line for the available colour depths.)