I recently went through heck and back to fix this issue, and want to help you!
Out of nowhere, my Roku Express wouldn’t connect to the WiFi it had been connected to for years. I searched these boards, called my internet company, called Roku, and finally got the solution:
Roku Express only connects to 2.4GHz internet. If your router recently went through an update, log on or call your internet provider and ask them how to split your WiFi into 2.4GHz and a 5GHz channel. When looking for your network to connect to, have Roku connect to your 2.4GHz channel and you will be back in no time!