Reseting My Iphone

Hey so I recently left my IPhone 7 plus with no battery for 3 days after those 3 days I went to go grab my phone and put it to charge it was charging for about half and hour. When I went to go check on it it wouldn't turn on I thought it was pretty weird so i tried using the same charger to charge my iphone 5 to see if maybe the problem was just my lightning cable or cube but to my surprise when I charged the iphone 5 it was charging like nothing. I then tried turning my phone on to see that nothing would come up not even a battery sign or anything. I searched up what to do in my situation, I tried forcing it to reset but nothing I tried everything but nothing seemed to work. My mom then told me to see if I could take it to the apple store so they can factory restart it but before I go ahead and make an appoinment I want to see if there is a cost or if the service is free.