What's stopping my iPhone powering on after dropping it?

So I dropped my iPhone 4 onto a wooden floor. Nothing cracked and there's no obvious damage but it won't power on anymore?

It was fully charged at the time.

If I plug it into a charger / Mac then the apple appears for 5 seconds and then it dies again - almost as if a check fails and it powers off.

I removed and reinserted the battery, just to ensure that no obvious power connectors were loose, but it didn't help.

Are there any other connectors that could have 'popped off' or components that I could have cracked?

Appreciate the help...

Al