Black screen after successful battery replacement

Long story but gives all background.

Replaced the battery and the SE started ok. Transferred data from the phone it was to replace and then it wanted an iOS update.

That worked fine but then it wanted to connect to iCloud to restore some other data.

For that I needed to get the password from my other Iphone which sadly had just gone flat. It was late, in fact wee hours 1am, so I went off to sleep with my other phone charging.

Next day the SE would not respond in any way.

Tried accessing via USB to MacBook no sign of life.

Tried restarts including the 15 second top plus home, no joy.

So reopened the case, that is lifted the screen once more to check if maybe the screen connector has popped off. It was secure.

Thinking maybe the battery had died or its connector has come loose I checked and it seemed secure.

Disconnected the battery and checked it has around 4 volts, still charged.

Put everything back together including the very very fiddly screen connector clip that took longest of everything to get just right and ended back where we started: nothing, no response.

Any ideas?