You talked about gaz tube.

If it is a LCD, no issue, the mosaic or the back-light is dying.
It could be a miscontact in the flat ribbon cable, but I don't think
(check it).
The back-light is generally fed apart with its plug.
If you open, you can check the back-light.
But I think you have problems with the data cable.
You will need a microscope or a viewer for that check.

The question : did it come suddenly or gradually ?

a écritÂ*:
The model umber indicates it is an LCD TV.