IPhone X auto poweroff after a year

I was activated my Iphone X on 29 January 2018. Its black out in between 28-30 January 2019 when its was unused in my self. It was never dropped, used very carefully. I was trying to power on in all method but i failed. Show this in apple store singapore in september 19. They said to change it because it's not powering. Why this happened? What is the solution?