Most likely PSU failure
However 1st try booting with just motherboard cpu and fan and one stick of memory and graphics card (if not onboard graphics).
Leave Hard drive and DVD drives disconnected make sure nothing in usb ports. It may then boot to an error message
If still same swap RAM sticks and retry.
If still same the PSU, mother board, or Cpu is faulty. PSU is cheapest item to replace first (Yes lights may still come on if PSU has failed (lights require very little power.