Everything started opening a Visual Studio 2010 Solution and getting the following error: ‘The project type is not supported by this installation.’
So I looked for the <ProjectTypeGuids> in the csproj file. Then I looked for the GUIDs I found in order to understand what type of project was missing.
I found a very usefull List of Visual Studio Project Type GUIDs on CodeProject by Yvan Rodrigues.
By the way, the C# project I was trying to open was an ASP.NET MVC 4 project type. I downloaded and installed the right package and then I was able to open my C# project without errors.