If your on a laptop try closing the lid, then reopening. Sometimes wires may be loose :)
Why Is My Laptop Sceen Red Instead Of Black When I Start Up. Red Flickering Areas Remain On The Screen And Fine Blue Lines Remain On A White Screen.?
This is usually a symptom of a defective display. As I write this, the "w" in "write" marked by a small line one of my monitors. Unfortunately, if you are out of warranty, this usually means a costy repair.