If you have ever received an error message in a .Net application that simply stated “Server Application Unavailable” you might find this article useful.
Also you found below message including above:
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.
If you run more than one .Net framework in your web server and use same application pool for different web applications with different framework then you will get such type of error message.
1. Go to IIS. Right click on Application Pools–> NEW–>Application Pool.
2. Now go to your site–>right click–>Properties–>Select the newly created Application pool at the bottom of virtual directory tab.
Hope it will help you.