|As OtheHill has stated, the computer should work fine with the new hard drive since it draws very little power , unless you un-intentionally jarred something loose or unplugged something and forgot to plug it back in.|
That's assuming you DID unplug the computer before fiddling around with any connections inside the case BEFORE you installed the hard drive, and at ALL times when you were fiddling around inside the case after that.
HP Pavilion Elite m9425f Desktop PC (home support page, USA)
m9425f Specs (your "here" link)
HP and Compaq Desktop PCs - Motherboard Specifications, IPIBL-LB (Benicia)
Integrated graphics using Intel GMA 3100
*Integrated video is not available if a graphics card is installed
•Integrated graphics using Intel GMA 3100
•Also supports PCI Express x16 graphics cards*
Make sure the 9500 GS video card is all the way down in it's slot.
The monitor must be plugged into the video port for the video card, on the bracket in a slot space farther down on the back of the case, NOT the video port up higher on the back of the case where most of the ports are, which will NOT produce video when there is a video card in the PCI-E x16 slot.
As OtheHill stated...
"If you have an add in graphics card, it may require a dedicated power connector as well."
Your model probably came with the 9500 GS video card. I checked to see how much capacity your power supply must have when the card is installed. It should work fine with your system's original 350 watt power supply, or any power supply with a 300 watt output capacity rating or higher.
If the 9500 GS video card has a power socket, there must be a connector from the power supply plugged into that socket, that fills the socket.
Make sure your ram is all the way down in it's slots.
When the 24 "pin" connector, or the two connectors - 20 "pin" and 4 "pin" - from the power supply are plugged to the 24 pin main power socket on the mboard, and the 4 "pin" connector is plugged into the 4 pin power socket on the mboard, when the power supply has AC power to it and when it's power switch on the power supply itself is in the ON position if it has a switch, there is probably an led on the mboard surface that is supposed to light up even when the computer is not running.
If that led is not lighting up, you probably have the switch on the power supply itself in the wrong position.