Clue as to what went wrong here? iPhone DEAD after chassis replacement
I'm a bit stumped at the moment, and unsure how to proceed.
: attempted, what seemed, an uneventful screen + rear chassis replacement on my iPhone 6 (16GB, iOS 11.4.1). No prior issues with the phone/screen- just wanted to freshen things up a bit. Using iFixit guides, I first replaced the screen and tested the new screen in-situ (working just fine), then removed the new screen and carried on with a full chassis replacement- being mindful to disconnect the battery flex before attaching/detaching the screen cables. Wrapped up the repair & closed up the phone, but on attempting to power on; - black screen/no startup sound/images...absolutely nothing.
: followed the battery of troubleshooting tests suggested online:
- care in avoiding long-screw injury (screws all initially labelled) on front panel assembly bracket ✓
- double-checked all screen cables properly fitted & in the right place ✓
- disconnected battery (then screen cables) & performed a power cycle ✓
- plugged phone in to charge for ~1hr (no response- no battery symbol/sound/heat) ✓
- disconnected battery & plugged phone directly into power source (no response, as above) ✓
- plugged phone into Macbook (no response- not showing in iTunes or anywhere else) ✓
- retested old screen (no response, as above) ✓
- checked all other connections (scraped phone down to the bones again) ✓
It just appears completely unresponsive/dead. Just can't fathom ?! I'm trying hard not to think that, somewhere along the line, despite being obsessively cautious and following the iFixit guide to a T, the logic board was inadvertently damaged- but I'm losing hope.
 Does anyone have any idea what might actually be going on here?
 Any additional ideas for troubleshooting?
 Any definitive test(s) that could differentiate whether it's a logic board issue vs something else?
 Any suggestions/solutions based on what you think might be the likely cause?
 Finally, are there any users particularly versed in these sort of issues, that I can reach out to?
Thanks in advance for help guys! I'll be truly grateful.
Is this a good question?