The cause can be bad memory in a laptop, some don`t boot if the case. The thing about the dv6700 laptops had to do with the nvidia 8400, or 8600 graphics chip alot of them were recalled due to a fault in the chips, they tended to fail under heat and the solder melted on the underside of the chip connecting it to the board. That was the most common fault with them. The result a black screen and no bios post. A re heat of just the graphics chip may fix the problem, but I would use a heat gun, pass it around the chip in circular motion about four inches away fron the chip no more than 15 seconds a pass four circular four or five passes should do it. leave it to cool about 20 mins if your lucky it may work, or the chip has died due to excessive heat build up over time of use. In that case its done for I am afraid Chrism.