Welcome to iMore! It may be the battery. Do you see anything at all on the lock screen while it's on the charger? Battery icon? Does it just get warm? Try charging it for an hour. If you still get nothing after an hour, take it to an Apple Store and let them check it out. If it gets extremely hot, don't charge it. Just take it to have it examined. The 6 Plus is almost 2 years old now. Batteries will start to go bad. Especially for heavy users like me.