|Impossible to say with any certainty without us being there. You will have to carry out a process of elimination to pin-point the cause:|
1. Try re-installing the video driver which you can download from your PC maker's website. If red line is still there, it's either a faulty video adapter or a faulty monitor.
2. Try a different monitor if possible. If that does not show red line, your usual monitor is faulty. If red line is still there on a different monitor, suspect a faulty video card.