Here's a link
to troubleshoot the startup of your notebook, from the HP online support site, and another link
that explains how to reseat the ram, which is worth a try. Has it exhibited any unusual symptoms, blue screens, etc. recently? It sounds like either some kind of hardware conflict, or a bad power supply.
Unfortunately, the nc6120 has its internal power supply on the system board and can't be changed separately. The part #s and removal/replacement instructions are in the service manual, link here.
This is a somewhat widespread issue, and if you're still covered by warranty, I recommend you get it in for service. Otherwise, you or a reliable tech could start selectively unplugging hardware and see if that is indeed the problem. See links to similar posts here
Best of luck with it. I hate to be the bearer of bad news, but have it checked out before you buy a new board.
I hope this information allows you to resolve this issue. If you need further assistance, please post back with a comment to this thread.
If I've managed to answer your question or solve a problem, please take just a moment to rate this post....thanks!