I accidentally pulled out the logic board socket when re-disconnecting the battery at this step. I came here from the iPhone 6 Battery Replacement page, where the disconnect step has this warning: “Take care to only pry up on the battery connector, and not the socket on the logic board. If you pry up on the logic board socket, you may break the connector entirely.“ I suggest adding that warning here as well, though it’s entirely my fault that I got sloppy.

And lucky, as the battery connection appears to be OK.

Btw, this was written in the comments for the 4S Power Button and Sensor Replacement, so if you see it anywhere else, caveat lector

I guess this is where I lost the screen. In any event... the power button now works, and I have sound, but no picture. I can't even punch in my PIN to reformat the phone and give it away.