If it also shows up in the BIOS, I'd assume that either the dotted line is actually some kind of physical markup from where it lands on the keyboard (have you tried cleaning it with a cloth?), or the screen is faulty, or it could also be the video cable- since you haven't stated any possible events between today and yesterday, I can only guess that it's from simply closing and opening the laptop. It could be that the video cable is not in securely. Also, try seeing if it appears on another screen connected to the laptop, like on a TV or monitor.
Welcome to Tom's Guide Forum, by the way.