Take everthing off so it is just motherboard, CPU, 1 stick of RAM and the PSU. Set everything else (GPU, other cards, drives, etc.) aside. Clear the BIOS as instructed in the mobo manual then booting again. If nothing appears on screen you can safely assume that something is fundamentally wrong with it.
EDIT: more specificaly there is something wrong with those core componants and it is up to you to decide what/if you are willing to replace in order to get this machine back up.