This is happening to me on my Roku Stick - not a smart TV. It doesn't happen all of the time, but I would guess that 3 out of every 5 days I need to unplug it and plug it in again for it to be responsive. Once it's restarted it works fine, but it's a pain to have to do this so frequently. It is up-to-date with all of the updates.
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If your Roku model has an "Auto power savings" item under Settings/System/Power, this will put your Roku device into a low power standby condition after a 20 minute period of inactivity, in which nothing has been streaming and no user interaction has been received, such as when your TV is off.
In this state neither its connection to your home network nor its HDMI signal to your TV are active.
Pressing a Roku control key (like the Home button) on your Roku remote (not Power, Volume, or Mute buttons, if your remote has them, as they act on the TV, not the Roku) should bring the Roku out of standby and back to full operation. Pulling the power for a few seconds to restart the Roku will restore full operation as well.
If you disable "Settings > System > Power > Auto power savings > After 20 minutes of no interaction" this should prevent your Roku from going into standby in the first place. The power savings are very small and for the hassle it causes most users, I don't think it's worth having this feature activated.
For further details, see this Roku information page: https://support.roku.com/article/6306527767575
