My computer won't post or boot

Hello

I recently decided to do a upgrade on my computer and so I put in a ssd and new memory cards, and new processor... the computer screen backlight comes on but screen remains black - I cannot even get into bios... so I put all original components back and same thing. I tried a new original processor- no luck. I tried new power supply- no luck. I tried new mobo - no luck. Any ideas on what it could be ? The CPU fan runs for a few seconds after hitting power button then stops for a bit then starts again and stays on...

Any help is much appreciated!