If you're having trouble turning on your desktop computer, it could be due to a faulty power button. To check if this is the case, start by disconnecting the power switch from the motherboard. Take a flat-headed screwdriver and tap it with the two pins where the power switch was connected for 1-2 seconds. If the unit turns on, the power button is likely faulty; otherwise, you can try the power supply clipboard test.
If your motherboard has an integrated power button, you can use this to turn on the system just as a properly connected power button would. To access this, you'll need to remove the left side panel from the computer case. The method may vary from computer to computer, but it usually involves grabbing a handle on the outside and releasing it after unscrewing it. After doing so, remove the cover from the front of the computer before removing the cover that covers the power button, and then press the back of the button until it comes out.
If you're using a desktop computer and none of these methods work, you may need to replace the motherboard. If you're sure that the wall power, the power supply, and the power button are working, there's probably a problem with the PC motherboard and you should change it. It's also worth checking that you're using the latest BIOS if you're building a PC and it won't boot up and, especially, if you're using an older motherboard. Once you've checked all of these connections, it's time to test the PC's power cable that connects to the power supply (also known as the C13 cable).
If your case has a reset button, you'll see that it's connected to the reset switch heads, near the power button heads. A faulty power cable is not a common cause of a computer not receiving power, but it does happen and is very easy to check. After opening up your case and checking all of these connections from the power supply to your PC components, check if there are any loose ones and replace them if necessary. Believe it or not, a faulty CMOS battery is also a relatively common cause of a computer looking like it's not receiving power.
You can use one from another computer or buy a new one. When your power button or buttons start wearing out or require more force to work, it may be necessary to replace them. This is especially true for laptop or tablet computers where accessing this connection between the button and motherboard is almost impossible. If your desktop computer isn't turning on but its motherboard light is on, one of these possible culprits should be checked.