The screen display goes from normal and gets darker

It's not all the time but it's getting worse. Its like the screen gets split in to 2mm rows and the very top is normal but it gets slightly darker as you go down so the bottom is quite dark. It has now started flickering at times, more when watching a video. Obviously being 3 years old it has been dropped lots of times, I have a good case on so just wondered what could be wrong. Thanks