If you've already changed to a different Wi-Fi network, try these steps:
If these steps above don’t solve the issue, you’ll need to factory reset your Chromecast, then set it up again using your new Wi-Fi network and password. If you're using Chromecast with Google TV, follow these setup steps instead.
Instead of factory resetting your Chromecast, if you're comfortable changing your router settings, you can change your new Wi-Fi network name and password in your router's settings to be the same as the previous ones. Your device should then automatically connect to the Wi-Fi.