You don't have to completely reinstall your OS. Use a boot floppy or CD to get to a
prompt. Type root=/dev/hda1 (or whatever your root partition is) and you'll get to a command prompt. You'll have to login as root with your root password then you'll get to something that looks like
From here, you should (depending on your distro) be able to type in xf86config which will allow you to reselect your monitor, keyboard, mouse, video card, etc. Answer the questions correctly and be sure to read each one carefully before answering. Once you've selected everything properly, you should be able to go back into X by typing startx.
This way you don't have to reinstall everything from scratch.