When you boot, there should be a recovery option in the bootloader. It'll be called the same thing as the what you would normally boot from, just with "(Recovery)" at the end. It should be the second option in the bootloader. Have you tried that? I've got no idea what it does (never used it before), but it's certainly worth a try. I think it should be like safemode in windows.
Another way to do it would be too boot from a live CD of your favorite linux distro (probably ubuntu). When it boots, mount the HDD that ubuntu is installed on. On the HDD, navigate to "/etc/X11". In that folder, there should be a file called "Xorg.conf". Open that file. Within that file, there should be one or more different monitor configurations. It's pretty easy to understand whats going on within the file. You'll need to find the configuration with a really oddball resolution, and either fix the resolution, or just delete the profile. I STRONGLY RECOMMEND MAKING A BACKUP OF THE "Xorg.conf" FILE BEFORE EDITING IT!!! This should work, but i'm not positive since I don't have access to a ubuntu machine at the moment. Note that if you can get in using recovery mode, then just edit the file in recovery mode. You'll have to use a "sudo" command to be able to edit the "Xorg.conf" file though.