Visual Studio 2013 is unable to open the Test Window

Deleting solution.v12.suo and solution.suo can help resolve the problem as well.

I tried to best answer (navigate to folder and delete it. %LOCALAPPDATA%\Microsoft\VisualStudio\12.0\ComponentModelCache) and it didn't work BUT, based on this method I found the solution which works for me :

navigate to folder and delete it : %LOCALAPPDATA%\Microsoft\WDExpress\11.0\ComponentModelCache

Hope I can help some people this way

This seems to be caused by a corruption in the Component Model Cache.

Close all instances of Visual Studio, then navigate to folder and delete it.

%LOCALAPPDATA%\Microsoft\VisualStudio\12.0\ComponentModelCache

Now restart Visual Studio and it should work again.