This is probably Denon protection circuit being activated. This can be caused by speaker connection short circuit, or electronics failure in the power supply board. For those who have warranty, take it to repair.
For those who do not have warranty, there are various YouTube video and blogs that teach DIY repair, which does not look easy to me.
My previous Denon AVR-3802 started to have this symptom after being used for roughly 12 years (power was cut when not being used).