Stock Web Projects don't have a project file - they are file based. However, they do have a Solution file that holds the Web site and that's usually stored in your Documents and Settings folder.
To reopen you can just navigate to the folder and choose Open Existing Web Site. Or the more likely way to do this look on the Recent Files list and the solution should be there.
Personally I no longer use 'Web Sites' but Web Application projects, which is a more traditional project that has a real project file and behaves more predictably. There are lots of reasons, but WAP projects are much more manageable as part of the development and deployment project.
To use WAP choose Create New Web Application. You can add Web Services to a WAP application (or Web Project) both for WCF or ASMX style services.
+++ Rick ---
>I don't see a .SLN file for the web service I created.
>
>So how do you reopen a Web Service solution/project?