Why is the computer not working after Assembly?
To say exactly without diagnosis difficult.With 95% confidence, I can report that it's in the BIOS setup.
You have embedded videos and external card - move the video card, reset BIOS with jumper, connect the monitor to the video input of the motherboard, go into BIOS, set up, specify what you're going to use external graphics card and only then stick a dedicated graphics card.
Can be different but there should look, so if the above didn't help-
1) Put the speaker and listen as he squeaks at startup.
2) Try with only one strip of RAM to start.