Ok, a couple of things. First of all, to clarify, when you first turn on the PC, you get the HP logo, do you get the POST screen where it shows hit F1 to setup or shows ram check, etc... white letters on black background? It can't boot if it's not finding a boot device "which is the highest possibility".
First, right when you get the HP screen, either tap esc or tab key, see if that gets you anywhere. If not, which I'm guessing then do this...
Unplug the power to pc, open it up, ground yourself to the pc metal frame and try to work on non static surface, don't need to fry anything inside. Now test to make sure all IDE cables (or other cables) are secure to the drives\devices, remove the CMOS battery for about 15 seconds and put it back in.Try to start the pc. I would also make sure you check to see if all fans are spinning up. It could be a simple loose IDE cable preventing the pc from booting, or could be a bad IDE cable. Could be other issues but let's knock down a few simple ones to be sure.
If you don't now how to do so, I can detail all this for you, just let me know, we don't need to cause further possible damage.