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In this scenario is better to start with a minimum configuration, the fewer components the better. Try to turn the system on with the CPU and the power supply cables connected to the motherboard; the 24 pins cable and the 8 pins cable. No memory, graphics card or drives yet.
Pay attention to what you hear or see on the motherboard. What LEDs are on and if there is any code in the little display on the corner. Also if you hear three beeps then add one memory stick to the blue slot closest to the processor and check if you get video from the motherboard’s HDMI or your graphics card.
It seems to be the motherboard is receiving power but you may test another power supply or get further information interpreting the motherboard’s behavior and status LEDs with the Technical Product Specification (pages 43-45)
This will give you something to start with. Additionally you can also perform a clear CMOS by removing the power and the battery from the motherboard for at least 30 minutes.