Power Mac G5 doesn't boot/black screen

I have a Power Mac G5 (Model No: A1047 EMC 1969C). My problem is that it doesn't properly boot anymore when powered up. When I'll press the power button, the white LED next to the button will illuminate briefly and the fans come on for very shortly and slow down again. The screen stays dark. Then the fans very slowly increase their speed until they seem to reach full speed. This takes probably around 5 minutes and the fans stay on unless I force my Mac to power off with the power button. The screen stays black the whole time. The white LED next to the power button doesn't blink. Any idea how to trouble shoot this?