I aquired a few Hp nv6400 laptops none of them power on .

To start check the power level of the battery with a multi meter, also get the right power adapter once you've eliminated that as a problem lets move on .This could be an host of issues any one of which can cause this so lets start with the basic. remove the laptop battery and disconnect the power then hold down the power button for 60 seconds. then reinstall the battery and plug in the power and boot the laptop. if it works it was a case of static discharge if there is no joy lets move on to the ram . Remove one stick of ram and try to boot the laptop if it doesnt boot then move that stick to a different dimm slot. and try to boot again if the laptop boots it means one stick of ram was bad if not then switch out and test with the other stick keep trying until you have tried each dimm in each slot . If you have no joy then we move on to theGPU. try pluging in an external monitor. If it works then your LCD screen is bad and will need to be replaced if it doesn't work then the issue is probably with the GPU

Hp 6000 series where bad for solder problems on the GPU. The solder cracks and you loose connection . There are several ways to fix it some better than others . What you need to do is reflow the solder . The best way to do it is to complete resolder the GPU which requires mad solder skills . My personal preference is to use a heat station and with a infra red thermometer, heat the GPU to 390 -425F. I maintain this temp for about 5 to 10 minutes. If a heat station is not available a hot air gun will work but a keep close eye on the temp not to overheat . If none of these tools are unavailable to you and you have a oven as a last resort you can try re flowing it in the oven at 375 . if that brings no joy then bump the temp up slightly till the board works . Im probably going to get a lot of negative remarks for suggesting this but if your in a pinch it may work . I have had success with this in the past .To bake a board it must be stripped down so that everything that can be removed from the board has been removed and it is very important that the board be placed in the oven as level as possible Put it on a cookie sheet with parchment paper bake it for 15 to 20 minutes at 375 then reassemble the laptop and try if you get no joy then bump the temp to 400 then to 425 or until it works. This will involve tearing down the laptop several time or if your lucky you get it on the first try . The reason you dont just start at 425 is that most ovens dont produce exact temps and in one oven 350 is actually 400 so its best to start low and work your way up. too much heat will ruin the board. Hope this helps