“Digital signal locked” just means that the connection between the TV and the Unitilite is working properly. So probably the cable and the Unitilite are fine.
Likely the issue is that the TV isn’t putting digital audio out, just an empty bitstream. I suggest you turn off and unplug the TV, wait a minute or two and then turn it on again. Hopefully it will now be back to normal.
You don’t need to wait for very long, but you do need to have the capacitors completely discharge so things start as if they had been off for longer. So a half minute or so should normally be enough. And if it doesn’t solve the problem trying a bit longer will never hurt!
With network things it can be longer, if you are trying to tell a remote controller or server that you have gone, and then you have come back again.