No picture after replacing LCD screen

I've done a few of these before and not come across this problem, but I have replaced a little bit more that just the screen.

I swapped out the charging port & battery, still with the phones cracked screen on and it turned on fine.

The phone now has a new screen on but it has BSOD. I've not put the display cable bracket back on at any point so I think that rules out the "long screw damage" issue that seems to cover most replacement screen BSOD.

The phone is recognised by iTunes but can't be accessed as it's locked. I've tried holding the on/off button and home button to reset it, no luck.

I'm stumped. The only mistake I know I made was the battery was still definitely connected atleast once whilst I was swapping over the display cables to see if the old screen worked.