@Ancomputersmn
Given the date of your post, I would have expected someone to respond to it before this -- but since that has not happened, I will take a shot at it ...
What you describing could be any of several different issues.
First off, you don't HOLD the power button, you press it briefly. Try that and see if it makes a difference.
Second, I'm presuming you don't even get POST info on an attached monitor, right? If so, the PC can't even do the preliminary POST work, and that implies a BIOS corruption problem.
Third, if you do get POST messages or an HP Logo on the monitor, is there a hard drive connected, and if so, do you have the drive activity lead connected to the motherboard? If so, do you see any blinking to indicate the drive is being read? If not, the drive could be dead -- and since you said you are trying to revive this, that could be the case.
Others may have more and better suggestions, but that exhausts what I can think of at the moment.
I am a volunteer and I do not work for, nor represent, HP