Is there an electrical short inside the computer case?
If the computer doesn’t power ON, even with minimal parts inside the case, power it off and unplug the computer from the mains. Then, closely inspect the computer case for any shorts. Usually, shorts are caused by screws that fall inside the case. These screws may come in contact between the motherboard and the case, and cause a short. The motherboard may also be touching the metal case at some place to cause a short. There could be other causes of a short and again, you should closely inspect the case. Hold the computer case and gently shake it sideways. If there’s a struck screw, it may come loose by doing this. If you see a screw at a place where your hand can’t reach, use a pair of long nose pliers or tweezers to remove the screw. If you tried to power up the motherboard on bench (see point no 8), and it powers up fine outside the case, an electrical short inside the case is most likely the cause of the computer not powering on. But at times, this may cause the computer to power on
