Hello @Berke_Tan1 and thanks for posting on the Microcenter Community Forum. You have a 8700K cpu which normally only supports 2666mhz RAM sticks. You have 3200 Mhz RAM sticks. Its not posting because the bios can't recognize your RAM speed.
There should be MEMOK switch next to the LED lights and its labeled MEMOK. Press that switch and hold it down for 3 seconds then release finger. Turn system off and back on see if the bios will then recognize your RAM speed.
If not, then you have to put 1 RAM stick into the first RAM slot closes to the CPU to get it to post to fix the issue.
One RAM stick is what you'll use in the first RAM slot near CPU.
You should then be able to post. At This point you can install windows and then update your motherboard's bios to see if it will work with your RAM sticks.
Or you can use another working computer ,if you have one; and download the motherboards bios file to a USB and then update the bios on your Z390 MOBO.
If updating the bios does not fix this you may have to go with DDR4-2666 RAM sticks due the 8700K.
See the link below on the Memory Specifications the 8700K supports.
@Berke_Tan1 If its been working for the past 2 years than you may potentially have a hardware issue (ex., wear & tear).
Will the computer post with just 1 RAM stick in the first RAM slot near the CPU?
You could have a bad board, bad RAM, etc. I'd recommend to bring it to the store for a full diagnostic.
Its $40 and they can find out what the problem is.
If its a bad board or RAM, if there is a warranty they can replace it. If no warranty, then at least you know what the problem is and you could do the repair/replacement yourself.
It would vary on if the new motherboard shows a new error.
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