Can you confirm that you're not getting any beeps from the system at startup? If no beeps, one possibility is that there's something conductive under the motherboard, like a spare mounting post or spacer (the little points of solder on the underside of the board are conductive, and anything touching them can create electrical problems). But perhaps first check that everything's seated correctly. Have you done any upgrades in the past? It can be hard to know how hard to push things together until you get a bit of experience.