http://en.kioskea.net/forum/affich-2415 ... witched-on
I had the same problem with my HP DV6599. On power on the screen would stay black, no POST, nothing at all on the display. The HDD would spin up but the HDD indicator would not come on and would not boot. The DVD drive would spin up and attempt to read a disc if one was in the tray but still would not boot. I found an answer on another forum. Unplug the power, remove the battery and then hold the power button for 30 to 60 seconds. Then reattach the power and attempt a boot. It worked for me! Just shutdown, put the battery back in you're good to go. Worked for me so definitely worth a try.