I would honestly suggest we start from scratch here. Something has gone wrong, and the best possible way to find it among the litany of things not working is to do this step by step.
Please consider removing your motherboard from the case if you've put it in, and getting a nice block of wood (or other suitably non-conductive material). We're going to start with breadboarding your system - so that adjustments are easy. You can connect power from your case for an easy button to try turning things on. You have a lot of things to potentially test, and the only way you're going to find out how badly you may have screwed up is by being thorough about testing each one in turn.
Please remove all cables and components you have installed. All of them. Please note to do this thoroughly, it is important you are treating it in a scientific manner - meaning isolate your variables. Do it one step at a time, and make careful notes if anything is in arrears at each point, including which point you're at.
Now, the first thing to do is inspect the motherboard. Do you see any large scratches? These can damage the traces and cause shorts or break functionality. Also, was the motherboard on standoffs, or was it directly touching the case (on its backside?).
Next, look in the CPU socket on the motherboard. Are there any bent pins? Is it clean?
Provided those are both good, carefully reinstall the CPU, and the CPU cooler.
Please carefully install ONE STICK of RAM, in the CLOSEST SLOT TO THE CPU.
Plug in:
- 12V ATX Rail (the widest connector you have on your power supply) to the motherboard.
- The CPU power connector.
- Your CPU cooler fan, on the header labelled CPU_FAN.
- The POWER Led from your case.
Check the power supply is plugged in, and check that it is flipped on. Do you get an LED on your motherboard that is glowing green, ideally? This indicates you have power flow to the system for at least the basics.
Next, try turning the system on. Listen for any beeps, and make sure that you get fan activity on the CPU cooler. Note any codes if the unit does not seem to be holding its "on" state.
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If you've gotten this far, we can probably expect that the motherboard is fine for basic use, and your power supply works. Your CPU is probably not damaged. So far so good.
Now, remove your RAM, and test each individual stick in this way. If two successive sticks do not work, move one lane over and try again. If that doesn't work, make a note.
If that's all good, try for the set, make sure you put them in paired lanes, as applicable.
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Alright. If we're still sitting pretty, your RAM works well enough to boot. That's a step in the right direction.
Now, we're going to try adding the GPU.
Plug in your graphics card, and make sure to provide it with any subsidiary power it requires from the PCI connectors from your power supply.
Try booting. Note any errors or if fans fail to spin up.
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Alrighty. Now try hooking in a monitor to your GPU. See if an image appears. If it does not, try doing this to your motherboard if it has video output. Sometimes it's a BIOS thing and you need to tell it specifically to use the GPU.
If we get an image, we're doing good. You can probably install the board back into your computer and reinstall these components as needed.
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Plug in your hard drives and other misc cables such as those from the front panel header of your case. MAKE ABSOLUTELY CERTAIN YOUR MOTHERBOARD IS ON STANDOFFS. If it is touching the case directly, not only do I have a good idea of what you did wrong, but you're doing it wrong. Make sure the PSU is switched off for this point in time.
At this point, you get to more standard troubleshooting.