Did your Rokus recently upgrade to version 9.3 of its operating software? Find out via Settings > System > About. Many people are reporting varied problems since receiving 9.3, although it seems to have slacked off of late.
Have you tried restarting your devices? Settings > System > System restart. That may be all it takes.
If still no network connection, try to reestablish your net connection via Settings > Network > Set up connection.
If STILL not getting network connection, try restarting your router. I know this doesn't seem to make any sense when other devices are connecting without issue, but it has worked for me on several occasions. When it comes back up you may need to again restart your Rokus and set up your net connections again (I've had times when I needed to and times when I didn't).
Good luck.
Oh, and one more thing. If you DID receive the 9.3 update, go back into Settings > System > System update and do a manual update check. This will make sure any of your channels that need to be updated for 9.3 receive their updates.
- Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
- Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T
- TCL Roku TV 43S425/C107X, Ser X000001R60KV, wifi