If I had the problem now, and do a reset, and then it worked, that would be proof that reset fixes the problem. But - it is working fine now, so there is nothing for a reset to prove. I am worried it will happen again, but on stage.
I checked - no phantom power, and no apparent power issues. Maybe I should buy another Helix and keep it as a backup on stage?
This is all good advice, and I really appreciate it!