I've had this problem with the unit (bought used over a year and a half ago) of it not giving any signal when turned on. Used to fixed it with a hard reset, but lately not even that works anymore.
I've tried to find info on the problem but I can't find anything.
I know it is the problem when I turn the unit on and it goes straight to the "flat mode" screen skipping the Line 6 logo part, I don't know if that helps at all.
The other thing I noticed is turning it on in test mode makes it work, but I can't control the volume.