iPhone 3GS will not powerup- Motherboard issue.

Hi, I was wondering if someone could possibly help me? I have recently started fixing iphones as a little hobbie, ive been given an iphone 3gs by one of my mates, he dropped it and it hasnt worked since, the screen was coming away from the casing and the volume and mute button are damaged. I stripped the phone apart and cant really see any major damage to the insides of the phone. I also own an iphone 3gs 16gb so i tried his motherboard in my phone and it doesnt seem to power up. I cant see no immediate damage to the motherboard in comparison to mine, Any ideas on what the issue could be or how i could go about fixing it? hes not a 100% if there is any water damage but i cant see any rust/corrison on the motherboard.

Thanks