BREADBOARD. ALWAYS BREADBOARD.
Take apart the entire computer again, fully. Place Mobo (no CPU,etc.) on a wood board or the Mobo cardboard box it came in. Does it have ONBOARD video? If so THAT is what you need to connect first, if not, then connect the GPU with HDMI. Seat ONE stick of RAM and the CPU/Cooler. connect from case ONLY the power switch, then connect the PSU ONLY to the Mobo. Power on.
Does it work? If it doesn't work at this point (no BIOS on the screen) then ONE of these components is bad / broke. If you see the display, shut down. Now add the second stick AS PER THE MOBO MANUAL as DUAL Channel (not simply the next slot). Power on, do you get BIOS and does it show that correct amount of Memory. If not, test again or replace defective memory.
Plug in GPU (if you have onboard video), boot to BIOS, and set the Video to use PCIexpress NOT onboard for default. Move connection from Mobo to GPU, reboot. Do you see BIOS? If not then reseat GPU and try again? If not then switch back to Mobo onboard video can you see BIOS? If not, then you may need to unplug the power from the wall, then remove the 'watch battery' for the BIOS for 10 minutes (NO LESS), plug back in batter then power to wall, and will reset the BIOS to factory (onboard would be default).
If all is good then plug in ONE drive, test to BIOS, is it detected? Plug in second drive, test, then DVD, test. When all components are proven in BIOS to be seen, THEN insert Windows DVD and install WIndows Only. Once done shut down. Unplug drives, power switch, ups, GPU. Fit in case, and plug back in GPU, PSU, get to BIOS yes? No? If no then reseat the GPU or check onboard GPU to see if it reset.
Again one thing at a time, DVD first, bios check, 2nd drive, bios check, boot drive, bios check then try Windows.
IT IS TEDIOUS, but it proves when a component does / doesn't work so your not pulling your hair out as you are now and no idea WHICH component isn't working.