@userOA6bemAxkc if none of the buttons work, we would recommend service for the device. You can find a repair center using the following link: http://www.samsung.com/us/support/service/location. Should you require further assistance please feel free to reach out to us! Thanks!
I too have just experienced the problem... I have had my phone about 10 months.
Model #: SM-G960U
Holding both [Power] and [Vol Down] for 10 seconds did reboot the phone and clear the problem.