Recently I faced this problem with my computer, and someone told me that the motherboard was faulty. So I already replaced it with a new one, but still it won't display anything. The fans spin and also mouse+keyboard power on. What could be causing this problem?
Well, considering that you tested a dedicated graphics card and it worked, I believe the iGPU is not working properly.
You can remove everything besides the essential before turning on your PC to test if the problem persists. If it does, the problem should be a faulty iGPU.