Screen faded to darker suddenly.

I have my Roku TV hooked up to my computer and it functions as a second monitor. While I was focusing on my main monitor, I noticed my brightness faded slowly to about half of what it was originally. I tried to fix the brightness, factory reset, unplugging from the wall, but nothing has worked. I can still see what's on the TV, and this isn't a black screen like some have reported with Roku. I also know that it happened automatically and faded slowly, meaning it was likely altering settings on its own. Can anyone help me out?