Help & troubleshooting for network issues, including connecting your device to your home Wi-Fi network, connecting to public networks, troubleshooting wireless issues & ethernet connections, and optimizing streaming performance.
The first steps are to reset/reboot your router, let router fully reboot, and then restart your Roku device (Settings...System...System Restart)
I second this ^^
An additional consideration if your Roku is an Express or a Premiere:
Roku models Express, Express+, Premiere, and Premiere+ cannot connect to the 5 GHz wifi band, only to the 2.4 GHz band. If you have one of these models 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.
There have been a number of reports lately of people that have routers supplied by their Internet Provider having their router's 2.4 GHz band disabled. They've had to have their Internet Provider re-enable the 2.4 GHz band, or even swap in a new router.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T