Why won't my iPhone 6s turn on?

Hello iFixit community,

So I've been having problems with a iPhone 6s and I'm wondering if you guys have any important information that can help me fix it. The phone was charging and I noticed it buzzed [I'm sure it was done charging]. My room was very cold since the window was opened and ended up falling asleep. I woke up about 4 hours later and noticed it wouldn't turn on even when I tried a hard reset, switching power outlets, and even using my Mac to recognized it but my attempts failed. I had a friend that fixes iPhones look at it and he was unsure about what to do - he tried using a lightning connector part from another 6s and reconnecting the LCD cable to test if it was the display, still didn't work. I'm not sure if its a dead battery, bricked, or even fixable. Please help; Any is appreciated.

Thanks in advance :)

-- PS: Before the phone died it would suddenly shut off out of nowhere and would only power on after I charged it or left it alone for a while.

--PPS: Just to let you know - The phone doesn't have any cosmetic damage or has sustained any risky falls that could have damaged it.