I am building a project in C# MVC4 using Microsoft Visual Studio 2012 and getting this issue. Whenever i add or delete any project files from explorer it does not reflects in solution explorer. Then I made practice to update files and folder from solution explorer itself to avoid such issue. But sometime it happens like if we get any new files/folder during SVN updates it does not reflect in solution explorer. To overcome this issue you can do following steps to resolve it:
1) Open Visual Studio and open the project.
2) Click on "Project" in menu and look for "Show All Files".
3) This option will show all files/folders added outside Solution Explorer.
4) Select all files and folder which you want to include and right click and choose "Include in Project"
Now it start showing in your solution explorer.
Hope this will help someone!!!