How to solve error message: "Failed to map the path '/'."

You don't have to reset IIS, you can just recycle the app pool.

restarting IIS solved the same issue in my case. Seems like something fails in IIS. Didn't worth my panic, either.

IIS 7.5, btw.

I had the same issue (MVC 4) under IIS 7. It turned out that the App Pool identity didn't have the correct authorization to the site's path.