First, to diagnose whether the whole screen does not work or just not the backlight, you can do this:
- Power up the phone, it will vibrate shortly to acknowledge that it starts.
- Wait for 2 minutes to let the phone finish booting, so you can be sure there "should be" something on the screen.
- Use a flashlight in a somewhat darkened room and light directly on the screen, by placing the flashlight head on the glas. If only the backlight does not work, you should see a dim screen image around the flashlight head now.
- If this did not work, try some other spot and make sure only little light escapes from the flashlight that does not go into the screen. Finding a spot that shows white on the display will work best for this, as this means the light can go through the whole display into the light dispersing mat behind it, disperse like normal display background light, and emerge from a larger area around the flashlight head.
[TODO: Find out how to test the screen backlight of a display unit.]
[TODO: Find out if the screen backlight (probably LEDs of a display unit can be replaced while keeping the screen.]
Procedure to exchange the screen:
There is an interesting discussion about typical failures of the screen replacement procedure. Mostly a gap between display and phone case at the upper edge of the display.