Roku operating system version 10.5 added a new "Power" item in the "Settings > System" menu for some of the latest Roku models. In my estimation, these power management options weren't very well thought out. If this is the cause of your problem, it has been reported that pressing the Roku remote's Home key should bring your Roku to life.
If you have a "Settings > System > Power" menu item, under "Auto power savings" see if deactivating "After 20 minutes of no interaction" keeps the Roku from shutting down.
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
