When running ADFS 3.0 On a Windows 2012 R2 domain controller, on a reboot, the Active Directory Federation Services service may not start up properly, and may not subsequently not service requests. The Services management snap-in shows the service with a status of Starting.
The reason for this is an error during the deployment of ADFS where a dependent service, the Microsoft Key Distribution Service, is not set to start automatically, causing ADFS not to start up properly until that particular service is also started.
The solution is quite simple: in the Services management snap-in, find the service called Microsoft Key Distribution Service, and set its startup type to Automatic.
After that, either start the service and then ADFS, or reboot the server.