Hi,
Sorry to hear about this
but I can give you a few troubleshooting options
Firstly, have you tried to hold Power + Volume down? I understand the button doesn't work, but it may be a case of the phone going unresponsive
Secondly, have you installed any applications that could of caused this issue? If so, remove them.
Last but not least! A simple factory reset could do the job if the other two options didn't work. If none of the three worked, I would highly recommend taking in your device to Samsung for servicing.
Good luck!