iPhone 6 is dead. Will not charge.

Hi.

So I have an old iPhone 6 that is not my main phone as I have an iPhone 7 Plus, but I still use it for temporary purposes and multitasking. The phone had been lost for around 2 days when it was accidentally placed in a rucksack which was then moved to a cupboard for 2 days. When it was found, the iPhone batteries were dead, and it was showing the no battery icon on the iPhone. So I decided to charge it, but 1 hour later, the phone did not turn on and kept showing the dead battery symbol.

I tried leaving it for longer than an hour like 6 hours and the same thing occurred. Tried using a wall socket instead of a socket extension unit, used and iPad 12W charger, same thing. I even tried cleaning out the charging port on the phone and when it was perfectly clean, tried charging again using the same actions that I did before, no luck. Was charging fine before, tried performing a Hard Reset, tried putting phone into DFU mode and connect it to iTunes, no chance for the iPhone 6. Any suggestions on how to fix this issue will be greatly appreciated.

Thanks.