IPhone 6s completely unresponsive

Hi, my IPhone 6s has been dead for three days now and will not turn on, charge, connect to ITunes, or reset! My phone was working perfectly fine before it died and once it died, I found that it would not recharge again. I've bought a new usb cable and wall adapter and leaving my phone to charge for hours at a time, even overnight, and my usb and adapter successfully charge my other devices so I know that they are not the problem. I have never had a problem with my phone not turning on again! My screen has been completely black for three days and no apple logo or charging logo appears. I have tried to reset my phone and even tried to put it in recovery mode but there is no response, even when holding the home and power button for five minutes at a time. My computer and ITunes does not detect my phone when connected and Find my IPhone says my phone is offline. I have also opened my SIM card port to check the LCI, which was not red, and I found no form of liquid damage. What can I do!?