PC is dead.

PC switched on, absolutely no sounds heard from the PC cabinet, whatsoever.
You should at least here a fan running in the power supply and at this point I would suspect that the
power supply itself had failed. However, the following could be checked before rushing out and buying a new one.

Check fuse in mains plug and that the wall socket is switched on.
Is the power switch in the 'on' position, this is located (if fitted) on rear of the power supply.
If you have a spare power lead swap it.
(Test)
No Luck with the above.
Disconnect the PC from the mains supply and open the PC cabinet to reveal the motherboard.
Check the following are true:-
The power switch connections from the cabinet switch to the motherboard are good.
The main power leads from the power supply are seated correctly in the motherboard sockets.
(Test)
No Luck with the above.
Remove all plug-in cards and disconnect the power leads and ribbon cables from all drives and auxiliary devices (card readers etc). Remove the memory. Make a note of where each lead goes and which way round the plugs fit.
You now have just the motherboard and processor with the power supply connected to the motherboard.
(Test)
If you now get a response from the power supply (i.e the fan starts) then one of the items you have disconnected is causing the problem.

* Remove the power and replace one of the disconnected items.
(Test) Go to * and repeat until the fault returns and the faulty item is identified. 

No Luck with the above.
The motherboard or processor is suspect.