iphone6 Plus not turning on or charging

Hi All,

Today, my iphone 6 plus' battery decellerated rather quickly while I was trying to charge it at work to the point where it shut off from the battery dying. I tried charging it, but it wasn't working and it won't hard reset, or turn on. Every now and then, I see the battery sign that shows it's dead, but trying to charge, but that doesn't last very long. Do you know what the issue is or how to fix? Please help!