iphone 5s black screen and unresponsive

iPhone 5s, 32gb. 6 months old. was fine until ran out of charge, tried charging it in car accessory and repeatedly turned itself on and off for about 1 hour until I pulled it out. unresponsive when plugged in to any power source now and all buttons are unresponsive, tried pressing home and lock button to restart but unresponsive, also tried charging for a while then pressing lock and home button to try and turn on, still nothing happens. when not in power source still unresponsive. no idea what caused it or how to fix it.