First of all, try a different power supply if you have one.
If you don't have a spare you can test your current power supply by unplugging it from everything, then shorting the green wire to any of the blacks with a spare bit of wire.
If it fires up as normal, then try reconnecting just the motherboard, if there's no improvement, check the seating of your graphics card, ram, processor, and any other cards you may have.
If this doesn't help, let me know and i can suggest a few more things to try.