Product:
Cognos Analytic 11.1.6
Microsoft Windows 2016 server

Problem:
When you browse to http://caservername/ibmcognos you get an error;

Service Unavailable
HTTP Error 503. The service is unavailable.

Solution:
This is in most cases is this that the application pool is stopped.

Go to IIS manager and start the ICApool.

 

More Information:
https://www.ibm.com/support/pages/request-failed-http-status-503-service-unavailable-when-launching-cognos-iis-gateway
https://www.ibm.com/support/pages/request-failed-http-status-503-service-unavailable-when-launching-controller