What are the old and new networks?
Did the Rachio stop working when you changed Wi-Fi? If so, did you set up the new network with the same SSID and password (otherwise you would have to reconfigure every Wi-Fi device) and try simply rebooting the controller?
Or, did you lose the Wi-Fi connection some days after changing Wi-Fi?
Can you see (with your phone, laptop, etc.) the Rachio-xxxxxx network being broadcast by the controller? If none of your devices show the network, try Wi-Fi reset:
If you still don’t see the Rachio-xxxxxx network, try a factory reset. Use Option 2 but don’t remove the controller from your app:
If you do see the Rachio-xxxxxx network but still get the error connecting, try from another device. A tablet or an old phone without a SIM should be fine, as long as it can connect to Wi-Fi, install the app and run it.
If you still have trouble, post details of what you have tried and what resources you have that may be useful (phones, tablets, laptops, etc.)