The OP's post sounded like it was due to a Roku update. Roku's software is notoriously buggy, untested (or, maybe a better way to say it is: the hardware Roku has licensed itself to run on is so broad that it can't be realistically tested on every revision of a tv model, etc. The bottom line is: it's unclear why there's so many update problems. Roku doesn't seem to be addressing it. It seems to be a status quo for Roku customers.).
Anyway, in some cases (the OP's case sounds like it's one), all you can do is factory reset. But, during the setup process, tell it you don't have internet. It will use whatever version of Roku software it came with. Then, never connect it to the internet (so it never gets an update.). In this way, you could continue to use your tv for antenna tv. And, you could buy an Android TV box (or Amazon Fire Stick) to use for streaming through an HDMI port. Basically "cut the cord" with Roku so your tv still works.
If you just bought it recently, and can still return it. Obviously that would be the smart move (I'd do that even if my tv wasn't broken, just from what I know now about what to expect with Roku TV). If it's still under warranty, open a claim with the manufacturer. Don't let them blame Roku. Your relationship is with the tv manufacturer, who has a relationship with Roku. (If they recognize a problem, let them stop putting Roku on their TVs. Roku will just tell you to talk to the tv manufacturer. Or, leave you to die on the vine without any answer whatsoever.).
Another possibility would be to investigate replacing modules inside the tv. There are videos and blogs explaining how people fixed various problems this way. I have a theory that these modules aren't all the same. I think maybe batches of them are made on contract to the lowest bidder. Some Hisense (or Onn, or TCL) use a module from "one lot". Others get a module from another "lot." The chipsets may be different. Roku isn't testing their updates against all this. So, some owners of that brand or model have a problem, while others don't. I think there's some kind of hardware anarchy happening that everyone (at the manufacturing and Roku level accept as status quo). It's hard to explain how this kind of "an update broke my tv" when it's not breaking all tvs. (Sometimes the solution is different too. Some people can factory reset and be fixed. Others find unplugging the tv fixes something. Others, none of that works. It has to be different "revisions" of tv models, the modular boards they use.).
"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.
