Q:

Application pool being stopped once browse the Tridion CMS site

Application pool of Tridion CMS site is being stopped once browse the Trdion CMS site. 
Its through below error 
Service Unavailable
HTTP Error 503. The service is unavailable.
Event Viewer Error Message 
"Application pool SDL Tridion has been disabled. Windows Process Activation Service (WAS) encountered a failure when it started a worker process to serve the application pool."

A:

The error you see in the event viewer tells that something is crashing the application pool when you start your CMS website.
One possible explanation of this could be that account under which your app pool identity runs has an expired password (see this answer https://stackoverflow.com/questions/10484155/windows-process-activation-service-was-encountered-a-failure-when-it-started-a).
If this is not the problem, see if you find any other useful logs (errors or warnings) in your event log

