The Nintendo DS uses a LCD (Liquid Crystal Display), hence the screen is basically a film with lqiuid crystal gel inside which acts as the display. If you put pressure on the screen, you can actually smear the substance and cause blurring or defects in the screen and potentially permanently damage it. So its best to avoid putting any pressure on the screen at all.
You may need to send the Nintendo DS in to Nintendo to get it repaired or you can try to do it yourself with could potentially void your warranty. At any rate, try searching google, theres several ways to fix/repair this problem.