Either the back light (a fluorescent) has failed or the inverter that furnishes high voltage for the tube. I'm pasting my suggested procedure below-
If it is affecting video only, the display's fluorescent back light or the power supply for it (inverter) is probably at fault and without some technical background and a couple of test instruments, you will not be able to do the repair yourself.
What you can do though is; unplug the set, let it stand overnight or at least several hours, remove the back panel and look for a smaller separate board with some larger components on it; it will have ferrite core transformer(s) on it and some medium-size capacitors which are cylindrical and mounted vertically to the board.

Before touching anything inside, especially on either board, allow several hours to be safe since the larger capacitors in the main power supply can hold a painful voltage charge for some hours.

The section most likely to fail and cause your problem will be one that you see associated more with the display than the incoming AC line; the latter is the main power supply and its failure would result in no sound or video.

If you see signs of overheating such as discoloration of the board material, it may not be repairable but if you can find a part number on the board, you might be able to buy the power supply from the manufacturer and install it yourself.
They are typically connected with non-interchangeable connectors.

If there are no signs of heat damage to the board, there may be companies offering to repair it as a subassembly instead of repairing the whole set which could be much more expensive.
If you choose to replace or have it done, the power supply still may not be at fault since a failure of the fluorescent, while likely to be less common, is the second possibility.