D-Prot appears to be a generic error message that does not indicate a single cause. I'd unplug it from the wall outlet then turn on to see if it goes away, otherwise opening the unit and looking for faults, or is that what you did when you wrote "no sign of burning"? Did you also look for failed capacitors, domed on top or leaking?
Since this is a DVD and amp all in one, I would look closer at the amp portion first, though for the power rating and size it is probably a class D amp which can be a little more work to reverse engineer and trace the circuit. That might begin with determining the amp chip and getting the datasheet, then measuring signal to its input pins, signal out of its output, and power input.
If there is an active protection circuit as there seems to be, it may have shut down the output so you could look for shorts including output transistors if the amp chip does not have that last stage of transistors built in.