1. It's possible you have a hardware failure, but it's also possible (I'd say, more probable since this happens so much) that your tv was broken by one of Roku's famous untested updates. You can narrow this down by doing the factory reset again, but tell the setup you don't have internet. It will use the software version it came with. You can then go to home>settings>network to setup your connection. Roku will eventually (automatically) check for an update in the ensuing 24-48 hours.
If your tv works again (then stops again). I'd be thinking about topics like class action. (They have a mac addr all zeroes problem due to a badly designed wifi board they've known about for years, and are still using without amendment. There's tons of reasons why people should be thinking about litigation.).
2. Since you're one of the fortunate Roku customers to have breakage occur while still in warranty, I would open a warranty claim regardless of whether it's hardware or Roku's recklessness.
TV manufacturers commonly point the finger at Roku, telling affected customers to take up their problem with Roku (a few moments reading this forum should show how pointless that exercise is). However, your relationship is with the TV manufacturer. You bought their tv, They have a licensed relationship with Roku. If they have a problem with Roku breaking in-warranty tvs, let them take it up with Roku. (This is another reason litigation should be on people's minds. Evading warranties is very serious business.).
Anyway, my finer points on this topic may seem out of place to you. If so, you could just do #2. But, it would be interesting to myself and others if #1 shows that an untested update did it to you. There's an awful lot of that going on around here. It could happen to you later, even after your black screen is fixed. You might want to rule it out for your own preparedness.
"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.
