@Nakul , look at your home>settings>system>update. That will tell you the date it was last updated. If the problem started then, that would be informative (maybe not helpful).
If you still have a warranty, open a claim now. Don't let the tv maker say it's Roku's responisbility. If you don't have a warranty, you're probably out of luck. Roku will tell you it's the tv maker's responsibility. There are videos about fixing a backlight problem (could be one of 2-3 boards. Or, could be the backlight strips. Or, it could be Roku's software "evolved" to a point it doesn't work with that tv anymore. Not fixable with new boards. That would be a risk, spending money trying to fix something that the software broke, or will with the next update. It may be better to cut your losses.).
You could try home>settings>system>advanced>factory reset. Tell the resulting setup that you'll setup internet later. That will bring the tv up in a lighter, seemingly different software. The antenna-tv UI is very different (and better, IMO). If you can watch antenna tv & stream through HDMI port, and the problem is gone, that might be a way to continue to use your tv. (At some point you could go to settings>networks and connect to the network, see if the problem returns. But, then you're exposing your tv to future updates which could break it. It could be good to leave it in the "dumb tv" mode and not press your luck.).
"People are often amazed at how much we’ve done with the number of engineers we’ve got." (Roku CEO Anthony Wood, Austin Statesman, Oct 4, 2019). "Amazed" is one way of putting it.
