I have been using a Roku Express for around two years. It is the original device being used on the same TV as always. I was watching a football game on FUBO, when the screen went completely blank. I tried to return to the ROKU Home page, but got a "Not Supported" message, just those two words. I saw a post about a similar issue, but that message the person saw was Mode not supoort", the one I see simly says "Not supported", no mention of mode or anything else. The unit and TV have been working flawlessly for the two years I have been with Roku. It did not say anything about a dropped connection, network problem, or anything else like that, just a black screen with two words, "Not Supported". Just the same, I checked to make sure my Internet connection to my router was solid, and it was/is, Then out of a surfeit of caution, I rebooted the router. I checked the HDMI connection between the Roku Express and my TV, and it was properly connected and solid, but to be sure, I disconnected it and reconnected it. I cycled the TV on and off, and finally powered down the Roku and then repowered it. The power light on the Express came on, but the screen still says "Not Supported". I logged into my Roku account, and everything there is fine. I can't get to the Roku home screen or any other screen, just the constant message "Not Supported".
The "Not supported" message is coming from your TV and indicates a problem with the video (HDMI) connection. Are you using the supplied power adapter and not your TV's USB port? When you disconnected and reconnected the power did anything come up on your TV screen? Did you try a different HDMI input on your TV or a different HDMI cable? Do you have another TV you can try it on?
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