Known solutions/workarounds for YT app not loading:
1) Check for updates (Settings/System/System updates/Check now) - try again
2) Change your Theme to the Roku Default (Settings/Theme/Restore default theme) - try again
3) Change your Screensaver to a Roku clock (Settings/Theme/Screensavers) - try again
4) Remove YT app, RESTART ROKU (Settings/System/System restart), Add YT app - try again
5) Network connection reset (Settings/System/Advanced system settings) - try again
6) Completely unplug power from Roku for 10-15 minutes - try again
7) Factory reset (Settings/System/Advanced system settings) <--- last resort
(NOTE: Do NOT remove the YT TV app - you wont be able to reinstall it for now)
