I just ran into the same issue. Roku box decided to update by itself. After restart, all apps will load, but all fail to display any audio/video when trying to play media. The menuing systems work, sound works on the audio signatures of various channels. This is the case for: Netflix, plex, hulu, prime, hbomax, amc and disney+. I've tried restarting the roku box multiple times from the menu, hard power off/on cycled twice, verified that all channels are up to date.
Any assistance would be appreciated.
From the issue report Tracker:
Model: 4800X - Roku Ultra
Software version: 11.5.0 build 4235-C2
Issue ID: TT-435-000.
Well, this is somewhat embarrassing. After futzing with this for a couple of hours. I happened to notice my receiver telling me "Roku 720p" instead of the normal "Roku 1080p" or "Roku 4K." I went into the Settings -> Resolution and manually reset it to 4K HDR, and now everything works again.
Hope this helps someone else.