Could be a main board of the system if no post. I'd attempt to boot it off of a live linux distro such as Mint or Ubuntu to see if it will boot off of the DVD rom, if the DVD Rom has lots of activity with the disc in it and still no picture then the display is shot, otherwise if it was just a bad hard drive, this live linux disc bypasses the need for a hard drive and the system will boot to linux and show on the display if it was jus the hard drive all along. *For the fact that nothing is displayed on boot and no error messages then your dealing with a main board issue or a display issue. If you can see if it will try to boot that Linux Live DVD in the DVD Rom, at least we would know that the main board is probably fine and its just the display.