Changed screen & logic board now not powering up

Hey Guys pls help if u can..

well my screen broke 8 months ago and because of a couple of emergency operations and a dodgy seller on eBay it’s taken me until now to fix. I thought it a good idea to buy a spares/repair phone as I wanted an oem screen. The phone I bought (2nd one after the 1st arse seller!) looked ok. The guy said the screen started pixelating then one day wouldn’t turn on anymore. Clean as a whistle no bumps or dings I deduced using my zero knowledge of mobile phones that this was probably a software problem, anyway I only want the screen and battery as the guy said it was running well until the pixelating problem.

My phone i’ll add was running fine, I just dropped the bloody thing and smashed the screen.

So I though to cut time why not just swap the logic boards over as the doner phones problem was hopefully in that area….plus the inside of that phone compared to mine was sparkling…looked new whereas mine was dusty and worn out looking.

Ok…swapped out the logic boards no problem, plus the screen and carefully as possible the home button to hopefully preserve the touch id. Put em back together crossing fingers and toes then shazam….. nothing, not even a peep… opened phone, disconnected and reconnected battery, still nothing… argghh… I really need a break, my first week of giving up smoking and have a lovely but very noisy toddler so please help meee!!