To debug a deployed Web service that's already running, you need to attach to the aspnet_wp.exe process manually. To do this in Visual Studio, show the Processes window by selecting Debug ’ Processes. Make sure the "Show system processes" option is selected, choose the aspnet_wp.exe process in the top window as the process to debug, and click the Attach button. Select which types of code you want to debug, and make sure that you've chosen the correct action to take when you've finished debugging. You can choose either to detach from the process being debugged or to terminate the process. Because terminating the ASP .NET worker process will terminate all ASP .NET applications running on that Web server, you should normally choose the former option.
Assuming that you have the Web service project loaded in Visual Studio, you should now be able to debug normally. Bear in mind that when the debugger is in break mode while attached to the aspnet_wp.exe process, all ASP .NET applications will be frozen and none of them will respond to Web page or other requests .