Roku express suddenly cannot find my wifi network, nothing fixes it

If your Roku does not find your WiFi network, this can be caused by a number of things.  And to make matters worse, some of them can be the result of recently-reported router updates. These have primarily been reported with routers supplied by internet providers (Comcast/Xfinity, Cox, and AT&T so far), but it's possible they could also occur with other internet providers or user-owned routers that have received similar updates.  If you have one of these situations, see "RECONFIGURING ROUTER", below. 

A) 2.4 GHZ WIFI BAND TURNED OFF
Some recent router updates have turned off dual band routers' use of the 2.4 GHz band. Some Roku models, notably Roku Express/Express+ non-4K models (with model numbers below 3940) and Premiere/Premiere+ (model numbers below 4620), are lower end units with single band WiFi radios that only support the 2.4 GHz band. They, and many other devices of all sorts that are 2.4 GHz only, can't even see the 5 GHz band, let alone connect to it. If you have a 2.4 GHz only Roku model and a dual band router with both 2.4 and 5 GHz bands, be sure the router's 2.4 GHz band is turned on.
 
B) WIFI CHANNELS 12-14
In the 2.4 GHz WiFi band, all Rokus support channels 1-11, as are used in North America. If your router supports 2.4 GHz channels 12-14 that are used in other regions make sure your router is set to use a channel in the 1-11 range and that you aren't using an auto setting that could select channels outside this range.
 
C) DFS CHANNELS
All Rokus with dual-band WiFi radios can connect to the 5 GHz wifi band in two ranges of channel numbers, 36-48, and 149-165. Channels between these two ranges are reserved as Dynamic Frequency Selection (DFS) channels and are shared with other uses such as military, radar, and weather. Only some of the newest Rokus using the latest WiFi processors can receive these DFS channels. Per the Wikipedia Chart at https://en.wikipedia.org/wiki/Roku#Feature_comparison, those models are:
• Streaming Stick 4K/4k+ (3820/3821)
• Express 4K/4k+ (3940/3941)
• Ultra/Ultra LT (4800/4801)
Many home use modems do not include the DFS channels but some do. If you have a Roku lacking DFS support that cannot find your WiFi network, try setting your router to NOT use DFS channels. If your router supports DFS channels and you can't turn them off, make sure you aren't using an auto setting that could select DFS channels.

RECONFIGURING ROUTER
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:

• Xfinity / Comcast → ( https://www.xfinity.com/support/articles/change-wifi-mode-admin-tool-xfinity-xfi ) with a walkthrough ( https://community.roku.com/t5/Network-Wireless-Wired-Connections/Error-014-30-stuck-for-three-days/m-p/749057#M21380)
  
  
• Cox → (https://www.cox.com/residential/support/managing-your-in-home-network-with-my-wifi.html)
  
  
• AT&T → (https://www.att.com/support/article/u-verse-high-speed-internet/KM1046172). You can select your appropriate router from the drop down menu.

If your problem was not caused by any of the above, a restart of the router (even if other devices are still connecting) followed by a restart of the Roku may allow the connection to be reestablished. Depending on your Roku model you'll find the "System restart" option under either "Settings > System > Power" or if you have no "Power" option, under "Settings > System", or if more convenient you can just pull the Roku's power for a few seconds and let it restart.