Hi,
Here’s a link to the [http://h10032.www1.hp.com/ctg/Manual/c04653884|service manual] for your laptop.
Scroll to p.71 to view the pre-requisite steps and then the procedure to remove the power button board. (the spare part number for the board is also on this page)
You may be able to test that the button is OK by using an Ohmmeter to check for electrical continuity through the button when it is operated, once you have disconnected it from the systemboard.
If the button is faulty, search online using the spare part number only to find suppliers of the part.