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.

Supply dead, fuse not blown - bad startup circuit (open startup resistors), open fusable resistors (due to shorted semiconductors), bad controller components.

https://www.repairfaq.org/sam/smpsfaq.htm#smpstsps

Until he puts in a new fifty cent fuse or tests the old one he won't know what is actually wrong if anything.

I run a retro repair workshop, most retro faults are dry joints on the solder.
90% are repaired the same day after a complete resolder and pot/switch service with wd40.