Pull all the EXTERNAL usb/e-sata/firewire connections out of the PC and see if it fires up!
Push all the power wires/connectors back into the hard drives/dvd/motherboard etc
You may have a dodgy/broken sata cable. Pull them out see if pc fires up.
Reseat graphic card/memory
Pull all the electric/connector wires out of the internal items (hard drive/DVD etc) and process of elimination reconnect them, see if pc fires up.
Pull the electric plug out of PSU for a minute or so.