You need to start from the bottom up. With everything assembled, you're just using trial and error. Do a minimal install outside the box with just CPU, GPU and, one stick of ram. Make sure the one stick is in the correct slot for that configuration. Make sure all case connectors are properly oriented with respect to pin one. Look for the embossed triangle and do not rely on the cable labels.
Try and boot. If it boots add another stick of ram. If no joy, substitute ram stick. If still no joy, try another GPU or onboard if available. Add one component at a time.
Sometimes you have to go back to basics. Even if you've built many systems.