There are a few possible reasons.
If you really mean "on" the screen, not "in" the screen, it could be something pressing against it - such as a plastic keyboard protector "skin" if your screen closes tight against your keyboard. Any screen cleaner will remove it, though you'll have to repeat the process periodically.
If it's in the screen itself, the most likely cause is failure of the control circuits for that row of pixels. (Each row has a control circuit, as does each column; the row and column control circuits cooperate to set the three color intensities of each pixel.) There is no inexpensive fix for this. If your laptop is under warranty, its manufacturer should fix it for you. If not, the cost will vary with the computer but it won't be cheap. If that's a concern, your best bet might be to find another computer of the same type that died for an unrelated reason, buy it for parts salvage value, and pay a service facility to swap the screens.