"wide horizontal lines" is a little vague. If you have another computer, or access to a friend's, plug your monitor into that one and see if you get the same thing. If you do your monitor might be going out. You could also try fiddling with the verticle and horizontal positioning of the image on your monitor -- sometimes if you move the image too far one way or another it can distort the image. If it's fine on your friend's computer it's got to do with something on your computer's hardware. And of course, you can always try jiggling the cables.
