I do have a suggestion, but I'm afraid we may require a few more details to give you a full answer. For instance, what desktop manager are you using? How did you change your resolution to an unsupported one? What exactly do you mean by your monitor "freaks out"? Does it give a specific error message, just crash, or just display incorrectly?
Given the vagueness, my best suggestion at this point is to check you xorg.conf file, and try to correct the resolution from there. The location of this file may vary for you, based on distro, but /etc/X11/ is a good place to look, or you can try searching for it with "slocate xorg.conf" if you have slocate set up. Good luck!