Could someone please tell me what part needs replacing?

Hello. I have an iPhone 5s and it is only 8 months old. One night I was listening to my music and the headphones and speakers were working fine. Before I went to bed I plugged it in and it started charging just like it always does. However, the next morning when I woke up I noticed that my speakers had stopped working and my phone did not recognize when I plugged my headphones in, essentially making my phone mute at all times. I could live with this. However, I used my phone all day, and like all phones, it started to die. When I went to plug it in it showed that it was charging and so I walked away. However, when I came back to it about a half hour later it still hadn't charged at all. I needed my phone and so I unplugged it and thought I would figure it out later. But my phone died all the way, I cannot turn it on anymore, and I have left it plugged in, says its charging, over night but it still will not turn on. I took it to the local Apple store to get a replacement and they told me my warranty was voided because of unauthorized tampering even though I have never opened my phone up. Anyways, I really need to fix my phone and I was just wondering if someone could tell me what part needs to be replaced. Is it the battery? Is it the lightning connector? I don't want to start playing around with the inside of my phone if it is not even the part that needs replacing.

Sorry for the long description.