DENON DRA-685 in 'protection mode' will not play

Does anyone have a clue to why our venerable Denon DRA-685 receiver is stuck in 'protection mode'? Everything fires up as normal: tuner, phono, cd, etc. but upon system turn-on it immediately locks into protective mode. No clue on our end, receiver has always worked fine. Any ideas as to problem would be much appreciated as well as any LINKS to a SERVICE manual, I cannot find one anywhere on the WEB!Thanks much, Martinluke!