My screen has a blue line in the middle.

@younes07 if it is a single pixel wide line it is a loose connection of a data line on the CoF that runs right where teat line is. A CoF is the "plastic ribbon cables' that connects the driver board to the LCD. Push on that cable while your TV is on. Right where your line is. If that makes it better, you know that this is a bad connection. Now, here is the bad part. You cant easily fix it. Sometimes wedging something between the case and exerting pressure on that, can temporarily fix this.

If this line is thicker, we will need to see what you see. Post some well focused pictures of that with your Question. Adding images to an existing question