Any ides why it won't 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 they told me that i hade to replace the whole logic board (since the GPU is solder to the Logic board). 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.

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

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 going. I assembled the whole mac (without a SSD).

When i connected the mac and pushed the power button nothing happened (i could hear the main hard drive spinning) but the screen would´t come on and there was 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 the original (intel Core i7 4771)?

3. It won't start cause its missing the SDD (I believe that you can order a iMac without a SSD)

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."