I have the same problem with HX Stomp. In case someone else who wrote in this thread reads this, did you have any kind of additional foot switches plugged in?
In my case it started happening sporadically, and with different firmwares. The unit would seem to be working fine but there would be no sound at all. It was obvious that it is not a hardware defect with ins or outs, or something with cables because as soon as unit is turned off, sound of amp (I am using HX Stomp in the amp loop) would be back. In the beginning the problem would go away after one or two restarts, but recently at soundcheck for a gig, it just stopped working and no matter how much I restarted there would be no sound. But, when I went back home, surprise, unit was working again. I have tried it both without and with Behringer footswitch that I am using as fs4 and fs5 switch.
So, at least in my case there happens to be a pattern. Sound cutting out ALWAYS happens when unit is in a bit colder environment - at a gig soundcheck or rehearsal. (THere is also slight possibility that unit is working fine in my house because of good power and has problems somewhere else, but it seems a bit strange that all those other places have bad electricity)
I have also read on different forum that someone has apparently learned it is a relay problem and this explanation seems to checks out. In my case, I have noticed that sometimes when problem appears tuner screen may pop out even if it was not activated, and then tuner screen would disappear but sound wouldn't come back. Also, issue seems to happen most often when unit is turned on for the first time in new environment. So it may be an issue with relay turning sound off as if tuner is activated. Also I have found an explanation that relay is turning sound off temporarily while unit is initializing so that there is no noise or pops and then turns it back on - and it is exactly one of the cases where issue seems to appear.
Also, some people have found it helpful to plug in USB connection, change input to USB and then back to analog - it supposedly forces relay to reset and sound comes back. I didn't have a chance to try that in my case as sound was back when I tried unit again back at home.
Also I wouldn't rule out a possibility that this issue is much more frequent than it appears - a lot of users (including myself in the past) won't write about it or use support ticket if simple restart has helped immediately. I started to bi worried only after the unit couldn't be revived when this happens.
I also have to rethink this situation. I had literary every generation of Line6 hardware and it was always solid. I simply can't rely on something that may stop working randomly. If this is the case with all Helix products, I may have to get something else. For now I have an old HD500 as a backup, but it doesn't have greatest ad/da conversion so I kind of dislike to have it in an amps loop in serial mode.
If I learn anything else, I will write back.
