I'm getting Error "Server Application Unavailable"

The webservice isthrowing the following error when i browse from IIS:

Server Application Unavailable

The web application you are attempting to access on this web server is currently unavailable. Please hit the "Refresh" button in your web browser to retry your request.

Administrator Note:

An error message detailing the cause of this specific request failure can be found in the application event log of the web server. Please review this log entry to discover what caused this error to occur.

In event viewer, I am getting the following message:

Failed to execute request because the App-Domain could not be created. Error: 0x80131509 For more information, see Help and Support Center at http://go.microsoft.com/fwlink events.asp.

I'm not able the understand the the problem actually is and how to resolve this. please help me.

I have found a very good article that worked for me:

https://askgif.com/blog/148/i-m-getting-error-server-application-unavailable-in-iis/

There seems to be a problem with your IIS installation, specifically, the Application Pool associated to your website. This could be because ASP.NET is not installed properly.

Please try the following:

1. Remove your website and application pool and recreate them
2. Install ASP.NET - run this command: %windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_regiis.exe–i as an Administrator.