Depending what you mean by "powering off", and not knowing what Roku model you have, this might be your problem:
Starting with Roku operating system version 10.5 a new "Power" item was added in the "Settings > System" menu for some of the newer Roku device models. At this writing they are Streaming Stick 4K/4K+ (3820/3821), Express 4K/4K+ (3940/3941), Roku 4 (4400), and Ultra/Ultra LT (4800/4801). You can check your model and model number via Roku menu path Settings/System/About.
If you have one of these models, the new "Auto power savings" item there 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. 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" it 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
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X