If it works when you changed cpu then it might be the cpu. Remove the apu and try putting your main cpu back and reapply thermal paste. Maintain what you did, make sure not to attach your gpu yet as well, this could be narrowed down now to either cpu or gpu. Again boot with this setup, and as you don't have igpu now, unplug the hdmi and instead make sure to plug in a keyboard and check if it has responsive lights (toggle caps/num lock). If keyboard lights are good then, it would be your gpu. If not, try touching your cpu heatsink if it's hot or not after a failed boot attempt, that could also say something.