VisualStudio has forgotten your debug settings. Right-click on your project in the SolutionExplorer pane, and select “Set as startup project”.  In the context menu, you might also want to select Properties|Configuration|Debugging|StartPage and set to path-under-project-dir/your-test-page.aspx

For example, if the project is “WebApp” and the test page is at “WebApp/test/Login.aspx”, then the start url could be “test/Login.aspx”.  The url could also be an absolute one using localhost or your machine’s true IP address.