Display darked out in 1 second

My Dell has very similar board layout as this one. It display for about 1 second after each power on and then darked out. Power off and on again give me another second but that's is. I shine a strong light on the screen in the hope that I can see a shadow of the display but nothing. With help from this site, I tear down the case but not able to troubleshoot because of my limited knowledge.

With the power supply board alone (not connect to any other board), tested it and all +5V and +12V output are ok and very stable. Found a pin labled "on/off along the DC out to the controller board (i.e. the one with the VGA port). But no matter I connect ground or +5V to it, the inverter still has nothing out.

Can anybody give me an idea how I can troubleshoot the problem? Thanks in advance.