What are the rest of your system specs? 400w is very low for a modern system. Though you would think you would at least get POST even if it crashed from low power later on.
The 8pin CPU power is only needed for more powerful CPU's such as high wattage quad cores and probably some of the high performance dual cores. Though it doesn't hurt anything if you use an 8pin even if the CPU doesn't need the extra power.
I will say this, if your system is powerful enough to require the 8pin connector then 400w is without a doubt too low. And if it does actually require the 8pin power then it most likely will not POST without it.
As Paul said there is no way of knowing if damage was done until you know you have proper power. I would think it depends on how the wires lined up that you put in there. But I would also expect that it would have caused some sparks and smoke if you crossed some lines. I have crossed DC lines before and it will let you know when it happens.
However with all this being said are you sure the PSU was the issue? Is it possible your board and/or CPU was bad?
Ultimate Edition Linux
AMD FX 8350
Asus Sabertooth 990FX R2.0
24GB GSkill/Corsair 1866MHz
EVGA GTX 970 SSC/ GTX 650 (PhysX)
SSD - OCZ V4 128GB/Kingston 120GB
HDD - 3TB/2x 750GB/500GB
CM HAF 932 Red
CM Hyper 212 EVO w/2x 120mm Red
Logitech X540 5.1
2X Acer 23" LED - Surround: 3840x1080
Microsoft Sidewinder x4 KB
"The significant problems we face cannot be solved at the same level of thinking we were at when we created them."
- Albert Einstein