Over the course of yesterday, many Google Home and Chromecast users reported issues with the devices. In the case of Home, commands were not being accepted while with Chromecast devices, casting would not work. The issue was reasonably widespread.
Late yesterday, Google’s Made By Google Twitter account posted that a fix would be rolling out over the follow six hour (i.e. overnight) to fix the issue.
We have a fix for Google Home and Chromecast. It will automatically roll out within the next 6 hours. To get back up and running now, reboot your device → https://t.co/CM4ov63F46. Thanks for sticking with us!
— Made by Google (@madebygoogle) June 27, 2018
At this point, this fix should have rolled out to everyone.