Could have been a problem with the downgrade. First thing to do is to restart both the router and the RokuTV and see if that fixes it. (Unplug the RokuTV for a couple of min if nothing is showing on screen). If the remote fails to turn the RokuTV back on, look for a physical button on the RokuTV (sometimes on the back or somewhere along the bottom left, right, or center below the logo). Some RokuTVs require a long press to turn on.
If the TV turns back on, then I would perform another System Update (Settings..System..System Update) followed by a System Restart (Settings..System..Power..System Restart). This second update will just check that the update and and your Channels installed the update correctly. (It should not upgrade to 10.5, but remain at 10.0 until Roku addresses the 10.5 issues).
If the above fails to load the RokuTV, then with the power ON (look for the power led lit up), press the Factory Reset button. (may be a pin hole and usually located on side near connections, but may be on rear on some RokuTVs). *** If you have Youtube TV installed, a factory reset will remove this Channel and you will not be able to reinstall Youtube TV. You will have to access it through the regular Youtube Channel.
Feel free to post back with an update or still need more help.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941