When you create a new project in Visual Studio 2012 using a blank solution. You add your web files, class files and any other structure to the blank solution. Then you try to build the solution and you get The project file “” has been renamed or is no longer in the solution. 

The issue arises because you did not add the class files you created to the web project.

Right-click on the project, choose References and make sure every project listed under References are included. Go to the next and do the same. That should solve your problem and the application will build.