Hey there Hayleigh783,
I'm sorry to hear that your Chromecast isn't casting! These kinds of issues can be tricky to troubleshoot on your own, so I'd be happy to help you out. The first thing I would do is forget the old Wi-Fi network and reset it back up. When you do, make sure both your Chromecast and Google Home app are on the same Wi-Fi network band, either 2.4GHz or 5GHz.
I look forward to your response, and if you have any other questions or concerns for me, please let me know!