Several of the one & two star reviews at Denon's AVR-S760H product page mention that their unit suddenly started going into "Protect mode" and shutting off. It sounds like you may be having that problem here.
About the only thing I would try before sending it out for repair is to disconnect everything from it and see if it still happens. Sometimes, something you have connected to the main unit starts failing and causes a weird impedance on one of the inputs or outputs (speakers). If you disconnect everything and it still shuts down, it's probably a bad board.