I have 3 streaming sticks in the household. This morning the one in the living room apparently wasn't recognized by the Roku server. Had to go through the entire setup procedure as though I had just bought another stick.
What may have caused this? Is this an early sign that the device barely 2 years old is failing?
I suspect your "problem" stick merely lost its wifi connection. Before I replaced an old Western Digital router with a New Asus several months ago, I had to reconnect several times a month even though other devices had no problem connecting. Since then my connection has been rock solid.
My suspicion is that at least some Rokus do not recover from net instability issues that don't cause problems with other devices. This possibly could be because the Roku is on all the time while most other devices are only using the wifi connection sporadically.
This has always worked for me to re-establish my Roku net connection when it has been lost or not performing well (if this works for you, it's easier than recovering from a full Factory reset):
If that doesn't fix it, try disconnecting the power to your Roku for a few seconds. When it comes back up, see if you then can set up the network connection.