Try
Take apart again, and remove the logic board, to kill all power to it.
Leave apart for a while to drain power in chips etc,
Re assemble BUT do not try to turn on until the screen etc is reconnected,
I have had the same problem on a few phones, it happens if you try connecting the screen while the phone is still poered on, it must have some sort of safty feature thats reset after disconnecting the battery
Keith