My iPhone 6 Plus won't come on

I have an iPhone 6 plus which sat in my cupboard unused for a while, the other day i went ahead and tried turning it on and it didn't start. charged it over multiple nights with different chargers, connection board and even cables, to my disappointment nothing worked. I am currently trying to repair it and thus trying to isolate the problem. Is there anyway to figure wether i have a dead logic board, charge port, battery, etc.

Really can't find anything on the net it terms of doing a check. So i decided to come here, thanks in advance.