Take a deep breath and recheck your connections. I know sometimes it feels like everything's in place, but a small mistake can make it unbootable. Try checking it again.
And while you are at it, please answer these questions :
1. Is the power LED turning on?
2. Are you seeing anything on the display? If not, are you sure that the VGA/DVI/HDMI cable you are using is working fine?
3. If power LED blinks, and if you are getting no output on monitor, then try connecting your monitor via the Motherboard instead of the GPU to figure out if the problem is in the GPU.
4. Post a picture of your motherboard so that we can review the problems, if any.
A pretty good build actually. Will do 1080p and 1440p just fine. A nice build on the budget. Now just somehow get it to work.