If I did I would power down and unplug the connection that issued the protect warning. I'd connect up a speaker of the appropriate load for testing in my rack or where the Denon is and then power up and see if it came back.
Given the story my bet is a wire pair shorted out. Given the distances and how speakers can be a few ohms you might see the short on the Ohm meter but that would not tell you much else.
I have a home audio system throughout my house. It is set up in 2 zones. Zone 2 is the Upstairs, main floor, basement and outside which is set to the denon B speaker. zone 1 is the living room with 5.1 speaker set to Denon A speaker. Ever since I bought the Denon it frequently goes into protect mode. Usually after a few hours of running on zone 2 but during the last cut off my speaker selection dissapeared? The receiver turns on but I cannot select any speaker or even see an option to select a speaker group......suggestions?