iPhone 6s plus crash

Yesterday my iPhone screen went blank for no apparent reason (it wasn't dropped or anything) but the sound still worked (When I turned it off it went "kaplunk" like it should do) When I got home I plugged it into the computer and it charged up and I was able to use the hotspot. But later I tried a restart and it didn't turn back on and is now completely dead

I suspect the battery needs replacing but I wonder if there's something else that caused this chain of events. Any ideas?