Most Samsung TV's are meant to last few years at most . Every TV I had has been Samsung and all developed problems after 2 year . Main problems are line appearing on the screen and dark spots on the screen.
I did have one that had same problem as you and took it to Samsung repair centre . They said about 6-8000 bht to fix and I said no thanks as can buy new for another 5000 bht on top of that .
Took it home and thought it was the backlight that was the problem so took it apart and could see where some parts were bust on the light strips . Then ordered same strips on Lazada for less than 1000 bht I think it was and then fitted them using a YouTube video. Put it all together and hey presto worked perfectly. Was pretty easy to take apart , apart from removing the edging from the white board inside was a bit annoying as the edges were sharp and cut myself trying to separate it. Very easy to put back together. If you have patience and time I suggest trying to sort it yourself.