We recently purchased a Roku Premiere+. When Roku is idle (i.e. over night or TV not on), the Roku periodically powers off. The only remedy is to unplug and plug in the power adapter. I'm not sure if the unit is defective, but definitely frustrating. We have several other older Roku models and Roku TV which are always on or available and don't exhibit this behavior.
Roku devices don't power off. But there have been reports of devices that lose their HDMI lock and the screen will go dark. There was a new firmware release yesterday that appears to have resolved the problem for some users, Check to see if you have 9.2.0 build 4806 yet, and if not force a system update to see if you can get it.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.