This is a duplicate of Cloud Sql Instance restart hanging for hours, but when I try to contact Google, they tell me I need to first ask on Stack Overflow, so here it goes.
My cloud sql instance became unresponsive yesterday (unable to connect using my normal credentials via mysql-client and getting 'unknown error' when I tried using the console) so I decided to restart it to see if that would help, since I couldn't find any better debug suggestions. That was over twenty hours ago and the db still shows as restarting in my cloud sql dashboard.
It seems that your Cloud SQL instance is stuck, maybe because a rollout that Google made, or something similar, we can't help you via Stackoverflow. However, I recommend you to open a support case and we will be more than happy to help you. You can specify my name if you want (Alejandro Riquelme), and I'll take your case.
If you don't have enough support level to open a ticket to our support platform, you can create a private issue tracker here specifying your Google Cloud Project Number and the link to this Stackoverflow question. If you follow this last process, you can specify my name too and I'll be there helping you, because I'm sure that we will need to perform some actions on your Cloud SQL instance to fix this issue.