It sounds like your router changed the wireless channel to one unsupported by Roku devices. On the 2.4 GHz radio, the channel number must be 1 through 11. If it changes to 12, 13 or 14, your Roku won't see it. For 5 GHz, the channel must be below 49 or above 148. The channels between those two numbers aren't visible on Roku devices. So log into your router and manually assign a channel for the wireless radios.
It's also possible your ISP turned the 2.4 GHz radio off. For some unknown reason, some ISPs have been doing that, and if your Roku is a single band WiFi device, then it's not going to ever see your WiFi.
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