I am working on an ASP.Net web services project, and, out of the blue, lost the ability to debug in Visual Studio. When I tried to debug my application that tested the web services, it could not load the symbols of the web services project. It was all very mysterious as nothing about my environment had obviously changed; one minute it was working, then it was not. So after much reviewing of project settings, cleaning, and rebuilding, what worked was to go to C:\inetpub\wwroot\MyApp, delete everything, then do a fresh rebuild and Publish. Voila, I could debug and hit breakpoints again.