It would looks like this was a transient issue as your website is up and running.
I would assume your website is hosted on Google Cloud Platform. There isn't much information to go on with. It would be good to know where it is hosted as it could be a VM being unhealthy, the region where the app is hosted might have some networking issue, amongst other things that could've gone wrong.
On of the first steps to troubleshoot this issue would be to ping and tracert your website, to check if it is indeed down and indicate where in the network it could have been failing.
Next you could verify your resources where your website is hosted and application logs to verify the timeframe where it went down and confirm the error output, most likely a 5xx if there is a network issue or resources are not available.
I will invite you to open a StackOverflow thread or a ticket with Google Cloud Platform Support in order to investigate further as Google Groups discussion are not intended to troubleshoot individual issue such as these.