What's the most likely culprit.

So i took my rig apart to do some cable management and when I put it back together it is refusing to start.
I am at a loss.

My Specs:
CPU: i5 2500k
GFX: Evga GTX 460 1GB
Mobo: Asus Sabertooth p67 B3 mobo
2X2 Gigs DDR3 ram
320 7200 RPM GB Hard Drive

My mobo led is turning on but nothing else will.
I don't think its the power supply because I plugged in the power supply into another pc (I just plugged in the atx power connector) to simply see if it would start up, and the fan started and lit up no problem.

It's a much, much older PC so the other parts are not compatible for testing purposes.

I'm thinking its either the mobo, or graphics card. I also plugged in the graphics card when I had the power supply plugged into the much older rig, the card wouldn't spin up.

My mobo doesn't have any internal graphics so could a faulty graphics card cause the system to simply not start, or could it still be a power supply issue?

Im confident its not a cpu or ram issue, and the mobo LED is bright green so it is receiving standby power but I guess it could still be a mobo issue.

Also i plugged in a mobo speaker to hear for beep codes, and there are none so im not sure what that means.

I am at a loss, and extremely frustrated any input would be appreciated.

Thanks