It's possible your router has changed the WiFi channel to one unsupported by Roku devices. Most likely your TV only supports 2.4 GHz, so the channel needs to be between 1 and 11. If it's auto-changed itself to 12, 13 or 14, your Roku won't see it.
Even if it's on 1-11, perhaps some new interference is causing a problem. The three best channels to use are 1, 6 or 11. These are the only three that don't overlap with each other.
Some routers provided by your Internet provider are set to turn off the 2.4 GHz radio and only use 5 GHz. If your TV doesn't support 5 GHz, then again it cannot be seen by your Roku. And even if your TV supports 5 GHz, there's a lot of unsupported channels in that band as well. For 5 GHz, the channel has to be below 49 or above 148. The channels in between are labeled DFS, and aren't supposed to be used in many countries. So Roku simply doesn't support them at all.
How do you change your WiFi channel? Without knowing the make/model of your router, I really can't say. It's going to be somewhere in the WiFi radio settings.
Dan
Roku Community Streaming Expert
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