When I scan for wireless networks, my Roku model 3900x does not find any networks, even though it was working a few weeks ago. The connection was lost over a couple weeks ago, and I've tried scanning since, but I'm still having issues. I have already tried resetting the roku, but no other wireless device can connect to the WiFi.
There are some router setting issues that can prevent establishing Roku wifi connections. And to make matters worse, some of them can be caused by recently-reported router updates. These have primarily been reported with routers supplied by internet providers (Comcast/xfinity, Cox, and ATT so far), but it's possible they could also occur with other internet providers or user-owned routers that have received similar updates. Even though these issues have been around for quite a few months now, it is appalling how many ISP support people are unaware of them.
From your description see if anything below helps:
Thanks to forum user @AvsGunnar for providing the links below to instructions for reconfiguring router settings for the listed internet suppliers. If you can't do this yourself, you may need to contact your internet provider to make these changes for you:
If your problem was not caused by any of the above, often a restart of the router (even if other devices are still connecting) followed by a restart of the Roku will allow the connection to be reestablished. Depending on your Roku model and Roku version number you'll find the "System restart" option under either "Settings > System > Power" or "Settings > System", or if more convenient you can just remove the Roku's power for a few seconds and let it restart.