Confirm that the power supply unit has the large plug properly connected to the motherboard, and also check that a smaller (usually 8 pin, but can be as few as 4) are connected to the motherboard near the CPU as well. Also, remove ALL cards. Check that your memory and CPU are properly seated, see if it will power on again. You can even strip out the memory as well. If the MB beeps 3 times on startup that just says "there is a memory problem", but you'll see fans spinning, and you will have eliminated a lot of other stuff. Then, one by one, add components back to the board, until something causes it to fail.
Also, if this is all a brand new build, you may have a bad motherboard or power supply. If you have an alternate power supply to test, you may be able to eliminate one or the other as well. Otherwise, it's going to be difficult if what I recommended doesn't work.
Best Regards,
Scott
MIET, MASHRAE, CDCP, CDCS, CDCE, CTDC, CTIA, ATS
"Everything should be made as simple as possible, and no simpler."
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