@amanueltesfaye this is a late answer and by now you most likely have your phone fixed already. I am just answering this due to a lack of description in the other answers.
It is most likely an issue with the display. I'd start by replacing the screen. Since iFixit does not yet have a guide for this, use something like [link|https://www.youtube.com/watch?v=UxFG72xbS2E|this video] to replace it. Replacement screen are readily available at many online stores.
It is possible that this is caused by a failure of the video circuitry on your main board, so for that we need to rule out the display. If a new display does not resolve this, then the issue most likely requires a boardlevel repair. Here is the schematic for components around the LCD connector
[image|3161640]
While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.