Amazon Fire Tablet HD 8”: Screen, touch, sound and power works as I could see what was on the screen at a certain angle with my own light source and slide the brightness slider which quite interestingly did absolutely nothing.
I reset the tablet with the power+volume button but that didn’t help. Haven’t done a factory reset as I assume this is a hardware issue. Worked fine for 7 months until I posted this question.
I assume that the backlight is broken and perhaps the only solutions are to replace the screen or to get a new tablet.
Posting this question in case other people get this specific issue and if there might be a simple solution to this.