This typically happens when your router is configured to auto-select the WiFi channel it uses (almost all are by default) and it has selected a channel your Roku cannot use. Unless you have the most recent version of Express (model 3960) it only has a 2.4 GHz WiFi radio. On this band, your Roku will only see and connect to WiFi using channels 1-11. If your router has selected 12, 13 or 14, it will be exactly what you have, not even being able to see your WiFi.
I can't tell you how to log into your router to change the channel assignment, and if your router is provided by your ISP you might not even have permission to change it. If you know how to log into your router, look under the WiFi settings for the channel assignments for each radio (it likely has 2.4 and 5 GHz radios). For 2.4 GHz, the best channel to use is either 1, 6 or 11.
Dan
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