While my experience is with Windows-based machines, the basic hardware is very similar and still follows the same (or very similar) startup sequences. With that in mind, it sounds very much like the mother/logic board is dead. Power may be going through the board, but it's not powering the BIOS. This means either something between the power supply and the BIOS died, or the BIOS itself died. Either way, you're looking at a new logic board.
If you know cars at all, consider this: if the power button is the ignition switch, the BIOS is the starter motor. Turn the key and the lights come on, but the starter won't budge, so the engine can't turn over. Same basic concept. Unfortunately, you can't just replace the BIOS as easily as you can a starter...
Unfortunately, it's a bit late now, but a faulty hard drive won't cause a problem like this. Take the drive out and it should still attempt to boot. If this is not true on Macs, it's because Apple made it that way by design...but I find that more than a little unlikely.
I truly hope someone else has a better answer than that...but based on experience, it's a pretty classic symptom.