phone does not start properly after replacing rear panel and battery

I followed the 3GS replace rear panel / replace battery guide.

Once complete I tried to start, nothing happened.

I connected to laptop via usb. Phone screen is illuminated and apple logo appears for approximately 4 seconds, the switch off, and the same over again.

I assume without being aware I made a mistake in re-assembly, or even broke something during the action.

Does someone can advise what component / cable / connector might be affected?

Obviously the screen works, and something electrical works.

Thanks