i have a dv6700 and am experiencing the same problem. i tried the power button trick and it didnt work but after a couple times i turned it over to see if the fan was working and it booted up immediately. i have no idea what it could be but im going to open it up and check the switch. i dont know if this will help at all but it worked for me.
When you try to boot it what does it do? Does the screen turn black, does it give you a BSOD? Have you called Hp's tech help to see what they have to say. I know big box companies tech support usually get's trashed for bad customer service but the few times I had trouble with my HP I called their tech support and they wer fast and helpful. That's more than I can say for Dell and other big box companies.