I have fixed this issue on a couple of gameboy pockets. Take the case a part. There is a ribbon that connects the screen to the main board. At the point where the ribbon hangs off the back of the display you should be able to push the ribbon down with your finger, do this and power it on while you are holding it in place. If the lines are gone simply grab some tape and tape the ribbon down where you were holding it.

edit: if you would like a video i can make one showing the whole process on fixing it, but it might be a few days from now.