iPhone 8 in Boot Loop after Screen Replacement

I have repaired many iPhone successfully over the past few years, but last week I began a repair on an iPhone 8 and accidentally severed all the flex cables from the screen (camera, LCD and home button) when trying to remove the screen. Today, my replacement front-facing camera arrived so I proceeded with the repair. All went well except when I accidentally connected the front camera flex cable upside down and attempted to power on the phone. I realised my mistake and corrected it, and now the phone goes into a boot loop when connected to a charger.

Firstly, does anybody know which of my two mistakes may have caused this (or if there are other possibilities as to what might have done it)?

Secondly, what can I do to fix it? I don’t have high hopes but any help would be hugely appreciated!!