Why won't it start

Hi

a month ago my iMac 27" i7 2Gb late 2012 hade a problem with its GPU (different color vertical lines all over the screen). I called apple support and told me that the i hade to replace the whole logic board. So i went online an bought a new logic board for a late 2013 iMac 27" i7 2GB, i missed that it was sold without a CPU.

http://www.ebay.com/itm/191489455454

So i bought a CPU at a local store (Intel core i7-4790K, LGA1150) original is a (Intel Core i7 4771)

When i started putting everything tougher i realized that the SSD i hade from my late 2012 would not fit my late 2013 logic board. But i kept on going. I assembled the whole mac (without any SSD).

When i connected the mac and pushed the power button nothing happens (i can hear the main hard drive spinning) but the screen won't come on and there is no "turn on" sound.

Why do you think my iMac won't start

1. The new late 2013 logic board won't work with the other late 2012 parts(screen, etc)?

2. The logic board won't power the new (Intel i7 4790K) CPU correctly, should i buy (intel Core i7 4771)

3. It won't start cause its missing the SDD (I believe that you can order a iMac whitout a SSD)

According to:

http://www.everymac.com/systems/apple/im...

the iMac was sold without a SSD

"*This model has a Serial ATA (6 Gb/s) connector for a 3.5" hard drive as well as a proprietary PCIe connector for an SSD. If the SSD is not installed at the time of purchase, the connector still is present and later professional or self-installation is possible."