Have you tried the Roku on another tv? Also try another device on the HDMI port to make sure that the port isn't the issue. If the Roku doesn't work on other TVs either then try to use a different HDMI cord. Try all of these things and let me know how it goes.
Bear in mind that when I press Back on the Roku remote control, the black screen is replaced with the normal YouTube display. So suddenly the HDMI cable and TV input have come to life? Seems unlikely.
Next time the problem occurs I plan to do the following.
The TV has three HDMI inputs, currently used for two Humax PVRs and the Roku device. I shall unplug the HDMI cable from the Roku device, then unplug one of the PVRs and use its HDMI cable to connect the Roku device. On selecting the new input on the TV, so that it is seeing the Roku device, I confidently predict the black screen will reappear.
Watch this space.
Add me to the list: Starting about 2 weeks ago, both of my Express 4K+ players have frequently exhibited the "black screen with sound" problem. This happens both at the start of using the Roku and also after the Roku has been in use for 30 minutes or more. My two Rokus devices are about 2-3 months old. My temporary solution has been to restart the Rokus, then re-connect with my WiFi signal.
I've been happily using Roku's since 2017 with no issues until now.
We need a fix for this problem or Roku should issue a recall of these Express 4K+ players.
Thanks for reporting this issue here in the Roku Community, @Sgooter1!
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All the best,
Emman