Power cord fails, now computer won't POST.

This build was working fine for months. Only one issue- when I would restart the computer windows would not start to load. Just a blinking __. When this would happen the power button would be non responsive even for a hard shutdown. So I would have to resort to the PSU power button on the back.

One time when I had to restart for a program I installed I went through this process but the computer would not turn back on. No power, no lights. After some troubleshooting I found out that the power cord was dead. I replaced it and now I get all lights, fans, and hard drives to turn on but the computer will not post. I get no error codes, even when booting with one or no sticks of RAM. The power button is responsive (if that's at all relevant). I have performed all steps in this thread already.

Is it possible to have a bad PSU and still get everything to power on inside the system? Is there any way to tell if it's the motherboard instead? Any help is greatly appreciated!!

PC SPECS:
Mobo: Gigabyte GA-970A-UD3 AM3+
CPU: AMD Phenom II X6 1100T Thuban 3.3GHz
PSU: Sigma 500W Power Supply (came with the tower)
RAM: Corsair Vengeance 8GB (2 x 4GB) 240-Pin DDR3 1600
Video: Sapphire Radeon HD 4650 512MB PCI Express x16