Web services error Exception type: InvalidOperationException Exception message: Unable to handle request.

Hi, 

     How to troubleshoot 'InvalidOperationException'  error created while loading web services page? 

Exception information:
    Exception type: InvalidOperationException
    Exception message: Unable to handle request.
 
Request information: 
   
    Is authenticated: True
    Authentication Type: Basic
    Thread account name: IIS APPPOOL\Test Pool
 
Thread information:
    Thread ID: 6
    Thread account name: IIS APPPOOL\Test Pool
    Is impersonating: False
    Stack trace:    at System.Web.Services.Protocols.ServerProtocolFactory.Create(Type type, HttpContext context, HttpRequest request, HttpResponse response, Boolean& abortProcessing)
   at System.Web.Services.Protocols.WebServiceHandlerFactory.CoreGetHandler(Type type, HttpContext context, HttpRequest request, HttpResponse response)

Thanks, 

Abhatt731