The machine doesn't do anything. No beeps, LEDs or spinning fans.
Check if your power cable is firmly plugged in on both ends (you would be surprised how many issues this resolves)
Check if the power supply (PSU) has a button to cut the power to the machine. Check if this button is in the position denoted with ON or I
Check if EVERY power cable is plugged in securely, reseat if necessary.
Check if the power button is installed correctly (consult your motherboard manual)
Try shorting out the pins you connect the power button to manually. If this powers on your system, you have a faulty power button.
Check with a device of which you know that it works if the power outlet you're using is actually providing power.
If this hasn't solved the problem, check if your PSU isn't dead. To do this, you can follow these steps:
Unplug your power cord and/or flip the switch on the back of the PSU in the position denoted with OFF or 0
Unplug every connector coming from your PSU (this is very important)
Using a bent paperclip, short out the green wire on the 24 pin header with any of the black wires, like so.
If you have a PSU that shuts down its fan under low load, or a PSU that is fanless, connect something small like a hard drive to it.
Plug in your power cord and/or flip the switch on the back of the PSU in the position denoted with ON or I
If the PSU does NOT power on (the fan/hard drive you connected should start spinning) after following these steps, your PSU is in all likelyhood defective. Request an RMA.
If the PSU DOES power on, then the problem is most likely your motherboard.
The machine does power up, but my screen remains black (no POST).
Make sure your BIOS version is compatible with the CPU generation you're trying to run in your motherboard, you can consult the manual or the manufacturer's website about this.
Check if EVERY power cable is connected. Auxiliary power connectors included.
Make sure the cable coming from your monitor is attached securely to the graphics card. Also make sure it is securely attached to the monitor itself.
Make sure your PSU is powerful enough to power your complete system.
Make sure your monitor works by testing it on a different computer.
If you have both a dedicated GPU and an iGPU, try your monitor on both the outputs on the graphics cards as well as on the motherboard.
Make sure all connectors and cables are plugged in securely, reseat if necessary.
Make sure your RAM, CPU and GPU are plugged in securely, reseat if necessary.
If the motherboard you're using has debug LEDs, check the error code and consult the motherboard manual to see what it means.
If you have a debug speaker connected to the motherboard, note the beep sequence and consult the motherboard manual or this thread to see what it means.
Try clearing your CMOS.
Try booting your computer with only a motherboard, CPU and one stick of RAM attached (if you don't have an iGPU, plug in your GPU as well ).
Make sure your GPU works (if you have a dedicated one), try it in another computer.
Make sure your RAM works, try it another computer.
Make sure the RAM is compatible with your CPU and your motherboard.
If the PC still doesn't POST after this, create a new thread and, as explained here, post IN FULL DETAIL about your problem. Make sure you include the following:
Troubleshooting steps you already did
Any additional information that may be relevant.
Credits to BlueScope819