Do you get anything on an attached display? If it's booting but silent, your last sound settings might have been digital-out. The internal speaker might not be getting the sound output.
Assuming it's not booting, I'd be wondering about the hard drive after 5 years in storage. Can you bring up the firmware boot picker by holding Option while booting? If so, try booting from an external drive. Try unplugging the internal drive, then booting. I've seen HDs go bad, in such a way that the Mac plays dead.
Also, the usual advice. Take an apple a day ... er, try a PRAM reset. PRAM settings may have become incomprehensible after 5 years without power.
No, I didnt bother to plug in the monitor, as I figured no point since no chime... I think its got the single video card which has DVI output which I then use an adapter to give me the standard 15-pin cable to go into my TV/monitor... I'll attach a monitor next time.
When I turn it on, I seem to hear the DVD drive and hard drive try to spin up... I have disconnected the DVD and hard drive, made no difference.
I also noticed if I removed all the RAM, the fans spin hard and making louder fan noise.. not sure if that means anything... though if I remove all the RAM, I read that the white power light should flash, but instead it remains steady all the time.
The keyboard I dont think gets any power, as the "caps lock" light dont turn on , and the mouse attached to it has no red light for its sensor underneath...
I have re-seated both the GPU card and the main tray at bottom holding the CPU and RAM and blown away as much dirt in the area as possible...
It's just weird that this worked fine when I put it away for storage 5-6 years ago... alas...
I've put in a good 3V internal PRAM battery too.. no difference.
As for PRAM reset, doesnt that require the keyboard to be active ,but I think they keyboard has no power when plugged in...