Hi @janast ,
Try shining a torch at an angle close to the screen when the phone is on and check that you can see images OK.
If so then there is a backlight problem.
Check that the screen flex cable is securely connected to the systemboard and also that there are no rips in the flex cable.
Did you disconnect the battery from the systemboard before starting the repair?
If not then you may have accidentally blown the fuse that feeds the backlight power to the screen.
I can’t find a schematic for the phone but in some other phones the smd fuse is mounted near to the flex cable connector. Worth a look anyway.