Won't power on after slight drop. Connections checked.

Hi,

My wife dropped her 3GS yesterday. Not a bad drop as it landed on a deep-pile rug, so I was surprised to find that it had powered off and would not turn on or charge. The power is definitely off as it is not recognised by the PC or iPod dock.

I've removed the screen and main board, checking the connections as I went, but none of them seemed to have come loose. I've put it back together and the problem remains.

Any ideas please?

Thanks

Update: My bro gave me a broken 3gs to steal parts from. I put the main board from it into my wife's phone. It now powers on, but only when the mains power is connected. As soon as the mains is removed, the phone goes off.