Problem with hp compaq 6710s laptop

Hello,

I have an hp compaq 6710s laptop and since yesterday worked like a charm, today won't boot at all. When I plug in the power cord or attach the battery it opens on its own, black screen and turns off after 2-3 seconds. Then it repeats tself. I left it turning on and off for 1 hour but nothing. I dissassembled it tried 2 different sets of RAM, removed the CMOS battery, removed HDD and everything else attached to it but still nothing. I even tried a solution with "hard reset" but nothing. Also I pressed the power butoon for half an hour or so but it kept doing its thing.

For a few months before today, on various occasions it unplugged by its own the modem or the lan card (it said 10/100 broadcom now can be removed safely). Can it be a matter of modem card shortcircuit that causes this?

Thanks