Sometimes your token from Google will expire.
Also open up your server logs, and see if there are any errors related to authentication. If there are, try recreating the token.
Remove the old token from the Google Auth SSO tab in the Admin Panel and create a new one. If the root cause was an invalid auth token, this should fix the problem.
While you can always look for the logs Metabase leaves on your server file system (or however you collect logs), if you are logged into Metabase with an admin account, you can also access them from the drop down menu in the upper right hand corner.