That sounds like a hardware issue with your TV. Roku doesn't manufacture the TVs, so you need to contact the company that built your TV.
Most TVs have some sort of backlight that provides the illumination for the screen. If that light fails, you get a dim, perhaps even unviewable screen. Plasma and OLED use different technology, but any TV labeled as LED or LCD is illuminated in this manner. It's a hardware issue, so the TV manufacturer is the one to provide assistance.
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