Hi,
Has the phone been dropped or gotten wet at all?
Try shining a torch at an angle close to the screen to check if there is an image at all.
It will be very faint so try it in a darkened room if possible to better see if it is there at all.
''If there is an image'' then there is either a blown backlight fuse on the systemboard (and there's always a reason that a fuse blows which will have to be investigated) or a problem with the backlight circuit in the screen itself.
If it is in the screen it cannot be fixed. The screen will have to be replaced.
''If there is no image'' then as a DIY repair all you can do is to temporarily replace the screen and check if there is now an image being displayed or not, to prove whether the problem was the screen or on the systemboard. You may have to do this as well even if there is an image being detected again to prove where the problem lies.
Here’s a link to the ifixit [guide|53186] guide which may be of some help.