Tear the build down, and put it all back together again. Double and triple check everything. It could be an issue with a stick of Ram isn't sitting in the slot correctly, even though you swear that you didn't touch the Ram... or brush against it.
Check everything. Check how the motherboard is sitting on the stand-offs. Check every power connection. Check how the heat sink is sitting on the processor, and check to make sure the processor is sitting in the socket properly.
Because you don't have another machine to test any of the parts with, and presumably you don't have another power supply to test with this machine... and you don't have a PSU tester, or a Ram tester, etc... so all you really can do is break it all down, and be methodical in making sure that it all comes back together properly.
Now, before you just take it all apart, and put it all back together in frustration... stagger the build. Put the BARE MINIMUM that you need back into a working state, to check to make sure that just those components. That means Motherboard, CPU, Heatsink, one stick of Ram, and a power supply. No other cards. No hard drives, no optical drives... just the bare minimum to show something on the screen.