Hello,
Take the board out of the case and power it on a flat non conductive surface, if you do not have anything the motherboard box usually works but place a few layers of newspaper. This will ensure there is nothing behind the motherboard causing a short, check behind and look at the dimm slot solder joints.
The next aspect to consider is the CPU socket as the memory is a direct link to the CPU. The Intel socket pins can mis-align or even bend during the install of the CPU, either due to the pressure of a heatsink / water block - for example someone that fully tightened one corner on the heatsink / block instead of gradually adjusting each corner in a cross diagonal pattern to even out the pressure or just terrible bad luck and the socket pins were actually bent from the start.
So you need to confirm / check that all the pins are 100% straight and nothing bent, also for dust and check the base of the CPU, the gold contacts should all be clean.
If all the above correct and it still does not work out of the case you will need some extra parts to help fault find or get things tested in another PC if possible.
