Had a bit of a scare during a maintenance window. Ran some updates on our SharePoint farm and after that one of the sites wasn’t coming up. Kept getting a 503 error. When I checked the event log, I found this error message:
An exception occurred when trying to issue security token: The HTTP service located at http://localhost:32843/SecurityTokenServiceApplication/securitytoken.svc/actas is unavailable. This could be because the service is too busy or because no endpoint was found listening at the specified address. Please ensure that the address is correct and try accessing the service again later..
A quick search led to this article, and when I checked the AppPools, they were all stopped.
Starting the AppPools fixed the problem.