Computer won't boot with a GPU in it?

[TaiGeXGaming]

I'll try to quickly describe what happens.  My friend was playing Diablo 3 with my GTX 680 in his system when it suddenly went black and wouldn't boot properly anymore.

 

We took the GPU out and it boots fine, we can see everything and it goes into windows.  However, if I plug a GPU into it (obvs with the power connectors too), the fans go haywire on max speed but it still boots, however it has no signal to the monitor and it stays black.  Thing is, we can hear it doing everything in the background (Skype booting etc) but we just can't see anything.  Another weird thing is that we can still plug the HDMI cable into the motherboard and the screen shows but is weirdly interlaced, which as far as I'm aware shouldn't work.

 

We've tried my 680 and a friends 670 which work perfectly fine, but we don't know what's causing the problem.  If we could get some help, we'd be really, really grateful!

[MrChavelo]

Try putting it in another PCIe slot

[TaiGeXGaming]

Try putting it in another PCIe slot

Tried that, didn't work.

[TaiGeXGaming]

Fixed it, all it took was using one of my own power supply cables (We have the same PSU) and it works fine now!  Actually really surprised, it wouldn't work for like 2 months and we tried all the complicated stuff first.