Black vertical lines on a monitor are a common sign of display hardware or signal transmission problems. The main culprit is often the LCD panel going bad, mainly when the lines are seen in all programs and never change. Also, a loose or defective internal display cable, which causes signal loss between the motherboard and the screen, is another very likely cause. On rare occasions, it might be due to the GPU failing to process the signal properly, where the graphics card cannot generate the images as it should. Pressing hard or hitting the screen may also result in vertical lines that are there to stay. Pinpointing the exact cause is crucial because issues related to hardware generally call for repairs or replacements.