Black line across screen on Dell Latitude E6430

I have been using Dell Latitude E6420 from 2 months. Everything was working fine. Suddenly, black line on top of screen came and not going back. I tried to to take screen shot of that line but it doesn't come in screen shot. Can anybody tell me how can i get rid of that line?

FYI: I'm using Ubuntu 14.04 LTS and i suspend my laptop for hours.