Integrated stopped working

If you don't have anything connected to trigger then it has to be something else.  I agree that standby light should be on, even if something in software or lack od signal prevents M100 to turn fully on.  Could it be something trivial, like bad power cord or loose spade inside?  Did you measure voltage at the end of the power cable? (try to wiggle cable and connections).  Are you sure it is pushed-in all the way?  Did you measure fuse with ohmmeter?  Is there any other fuse inside? (be careful - disconnect power).  I'm just grasping at straws.

I agree with millercarbon that it might be bad electrical connection.  I have Benchmark DAC3 that suddenly stopped receiving signal from remote.  I opened it and found that infrared diode was poorly soldered on one side.  It was side of ground plane that required more heat to solder properly.   You can try to gently shake/twist it.
Samac has good point that circuitry, including stand-by light, is controlled by micro, that possibly needs reset.  Good luck, let us know.

Perhaps "trigger circuit" doesn't work.  Either something is connected but not driving, or something is connected by mistake, like 3.5mm headphone jack.

Try different power cord per turnbowm suggestion.   At this point anything is possible.  As for "unplug and reset" you should leave it unplugged for 15min or so.

@s7horton   Is it possible that it is in standby mode, but the bulb is burned-out?  Did you try to play something?