Hi @henlomate
It could be your back light circuit not working. Shine a torch onto the screen after the startup chime and see if you see a faded Apple logo on the screen.
If you remove the back cover you should be able to see the LCD connector as per the image below
[image|2559392]
Make sure you disconnect the battery first before touching the LCD connector to prevent blowing the LCD circuit accidentally!
Have look at the connector itself to see if there is any burn marks, missing pins or blown components nearby.