Powerbutton reacts, but doesn't start G5

After having being shut down a couple of weeks, my powermac G5 dual 2,3 (PowerMac11,2) won't start up.

As seen on this video http://www.spildaftid.dk/peter/G5_error.... the button reacts with a click-sound and the red diode, but nothing happens. You can see on the video that when I try the third time, I keep the button in, which results in the white power light keeps being on, but nothing happens after that (I tried to let it be on for five minutes).

There is no startup sound, no sound from HD (which is only 4 months old), nothing on the screen (and can't obviously start from dvd or external drive), the internal battery is also only around 4 months. The white light doesn't blink.

I have tried zapping the PRAM and pressing the PMU w/o any luck.

TIA,

Peter