iPhone 3GS dropped, working fine now it doesn't power on
Fixing an iPhone 3GS for someone and they said they dropped it (cracking the screen) and it was still working and they could slightly press the digits to their passcode but then the screen went black. I replaced the screen assembly on it and plugged in into a charger and it still hasn't booted up at all.
What could be causing this issue?
The internals of the phone don't look damaged and I made sure all the connectors are connected interally.