Custom PC Black Screen Help

[AxelCeee]

After building a custom PC, I had it fully working for a few months, then one day I got a black screen. Here are the things I have tried so far.
1. Changed the Hard-drive
2. Re-seat ram both ram sticks, and used both separately to make sure on of them wasn't working.
3. Reset CMOS
4. Unplug and re-plug everything.
5. Took everything out except the PSU and Motherboard/CPU, and the motherboard beeped.
6. Installed RAM and also beeped.
7. Held power for 35 seconds(Apparently this worked for some people).
I have just replaced the GPU,MOBO,HDD,RAM, Thermal Paste and even replaced two fans. The only thing I can say about the black screen was that my PC was running "fine" and at safe temps, I made another thread to make sure they were safe. I went to turn it on and got a black screen with a loud constant buzzing beep. I let it sit there for a couple months and now I'm ready to attempt to find the issue here. I'm suspecting the PSU, but I need some more experienced advice, thanks in advance!
------------SPECS-----------
CPU: Athlon 860k
GPU: GTX 750 Ti
HDD: Seagate Barracuda 1 TB
PSU: EVGA 400W
CASE: Rosewill Ranger-M
RAM: 2x4GB Crucial Ballistics Sport
MOBO: Gigabyte F2A78M-D3H

 

[smorizio]

start with a known good power supply. with the x chu you need a good power supply and a good video card. with a known good power supply if there no post then try a new gpu.if there no post it could be the mb or cpu.

 

[AxelCeee]

Thanks Ill do that, I'm gonna buy 500-550w PSU, then see what happens!