Question PC wont start

Jul 29, 2019
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So i built my first pc and everything worked fine the forst day. Second day ot wouldnt turn on. Trouble shooted everything i could and ended up buy a new psu and a new motherboard after the new psu didnt do the trick. Still nothing with the new motherboard. On the old motherboard the ethernet light would come on but the board wouldn't start so i thought psu then after getting the new one and the same thing happening i thought it had to be the motherboard got a new motherboard and still nothing. What is going on. Is it the cpu? The case? I'm beyond lost.