If you have access to another graphics card, to borrow, or that you have already laying around, I'd definitely try that as well. Certainly the power supply is my first concern, as always, but I see an awful lot of graphics cards that just seem to bite the dust when transferred to a new system, especially if they have some miles on them already, and the reason for it is beyond me. I have no answers for why this happens when they were working perfectly fine before, but I just know I see it, often.
So, check for bent pins on the CPU. Verify it's not simply a power supply issue. Try a different graphics card if possible. If everything else checks out, all that is left is generally the motherboard but I would also try to get it to post without any drives attached. Disconnect both the power and SATA data cables from all storage devices. Sometimes a bad drive will give the system a serious case of the "ain't gonna do it's".
Also, I know you said you tried one stick of RAM in every slot, but, that's really not a good method.
What you NEED to do, is install ONE memory module in the A2 slot, which is the second slot over from the CPU, and THEN, with the power off, do a hard reset of the BIOS as follows:
Power off the unit, switch the PSU off and unplug the PSU cord from either the wall or the power supply.
Remove the motherboard CMOS battery for five minutes. During that five minutes, press the power button on the case for 30 seconds. After the five minutes is up, reinstall the CMOS battery making sure to insert it with the correct side up just as it came out.
Now, plug the power supply cable back in, switch the PSU back on and power up the system. It should display the POST screen and the options to enter CMOS/BIOS setup. Enter the bios setup program and reconfigure the boot settings for either the Windows boot manager or for legacy systems, the drive your OS is installed on if necessary.
Save settings and exit. If the system will POST and boot then you can move forward from there including going back into the bios and configuring any other custom settings you may need to configure such as Memory XMP profile settings, custom fan profile settings or other specific settings you may have previously had configured that were wiped out by resetting the CMOS.
If it will still not POST. Then power off, and move the memory module to the B2 slot, which is the FOURTH DIMM slot over from the CPU, closest to the edge of the motherboard. A2 and B2 are the primary slots to be used on any dual channel consumer motherboard. Some boards designate them as DDR4_1 and DDR4_2, or they flip the names of the slots, A2 closest to the edge of the board and B2 being two slots over from the CPU, but those two slots are ALWAYS the primary slots and you should use one of them. It's generally recommended to use the A2 slot with a single stick and population rules always say to use both those slots with two sticks installed.
If the system EVER doesn't want to post with those slots populated, AND if the problem is memory related, then there is a problem with the motherboard, the CPU or the mounting of the CPU cooler which can in some cases be overly tight, or tightened unevenly at the four corners, or one side tighter than the other, which can cock the CPU in the socket and cause all sorts of problems from shorting pins to breaking the connection on pins, all of which can have a direct impact on whether the memory subsystem works right or at all.
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