Plenty of factors in tampering can cause this issue, and the most frequent are listed below.
remember to disconnect power source before any repair.
1) The Connections to the screen are loose: re-remove the screen, and attach it again. make sure you hear a click with each connector you apply. Should you have access to it, i would also suggest cleaning the connections with 99% isopropyl for a longer lasting device.
2) The Flex Ribbon has been damaged: the stress put onto the flex ribbons can not only have the chance to tear the ribbon, but more commonly, partially sever the circuit attached to the LCD/Digi itself. (this is not a visible damage, and is actually inside the LCD assembly. accidents can still happen. In this case, replacing the screen should fix the issue.
3) Inverter Coil Failure: Though most are attached to screens, some are moduler. A small board that the LCD flex attaches to, which processes the Backlight. replacing this may solve your issue.
4) Backlight control failure: Low quality screens, lack of Antistatic protection, and use of metal tools can cause static discharge which damage the motherboard components. A much more advanced repair, replacing components in this circuit should fix the issue.
5) Missing/damaged components in the related circuit: There are hundreds of components on a board. using the wrong tool in the wrong spot, slipping with the drivers or even a drop, can cause failures on the Screen. Manually scan the board for damaged or missing components in the areas you have worked, and replace as necessary. (A microscope is very helpful attempting this)
6) Motherboard damage: If ALL else fail, you have a perminant failure in the PLB, and need to replace it.