If there is no power at all, such as no lights at all. Then it has probably had an internal fuse failure, or something has gone in the power supply of the unit.
Make certain that any Blu-Ray player connected is putting out the right signals. Sometimes they can have the option to "mix" signals for certain TV's. This however will kill the signals needed for Atmos.
Try setting up speakers through your TV. (I suppose your Tv is wifi in to tv) Tap menu on tv remote. Go to network. Tap wifi, then follow TV's direction.
Power on the Harmon Kardon by pressing and holding the power button until it beeps.
then, press and hold the Bluetooth button (on H.K. Speaker) until it beeps.
Make the Samsung UN46EH5300 Bluetooth to be VISIBLE.
The H.K. will emit an audible when successfully paired.
Either track down the original remote for the model, or get a universal one, try and get the type that searches for the code automaticly otherwise you will be hunting around for the remote code that is correct for it.
That shows the unit is switching them off somewhere. Perhaps on different modes or surround settings.
If you have a DVD marked THX that will have testing settings for the system. Then you will be able to found out if a setting on the amp is responsible for the loss of channels.
Have you checked that both appliances are compatible with ARC HDMI? If they are then it is probably a setting problem. You should check that the socket is set up on both devices to act in the right way.
