If you gave Apple the serial # of the computer, they would have sent the install DVD (or CD) for whatever OS the Mac originally shipped with in 2004. Probably Panther 10.3. But it should be the right disc.
Why do you think it's Leopard? Leopard wasn't around in 2004. Did you order Leopard from them? If you did -- and I'm not sure it's even available from Apple Phone Sales any longer -- it wouldn't have been the grey, machine specific disc.
You may need to go back and check the connections to the drive. Or it might be a failed SATA controller to the drive. It might not have been working when they disposed of it. Any way to find out?
Try holding down the option key then power on. This brings up the startup manager. Click on your dvd. Click on right arrow key.
At least, this will give an indication that you machine is alive.
For more information
Sometimes if volume doesn't boot in Startup Manager (what you get when you hold down the Option key at startup), you need to reset the Mac's PRAM, NVRAM, and Open Firmware. Shut down the Mac, then power it up, and before the screen lights up, quickly hold down the Command, Option, P, and R keys, until the Mac has chimed twice more after the powerup chime. Then, before the screen lights up, hold down Command-Option-O-F until the Open Firmware screen appears. Then enter these lines, pressing Return after each one:
"The reset-all command should restart your Mac. If so, you have successfully reset the Open Firmware settings."