What most people forget it is that the files of visual studio are just text files, that have some peculiars configurations that will show to the program how to open it. that is, we can change this because it's just a text in some file in there in your project folders.
Well, knowing this, what we have to do is very simple!
The first step is knowing what kind of project it is this project that stay unload. (for example: Class Library)
The Second step is create a new one (Class Library) because you know that your visual studio will create a version supported by himself. Unload this one and click in "Edit csproj".
It's in this file that we can found the configuration that tell to VS how this proj will be loaded and his name is ProjectGuid, this serial number has a variation according the type and version of project.
Now, look at your "ok project", copy the "ProjectGuid" TAG, paste on csproj that unloaded, and pay attention to the little differences and make this files almost equals, except for the tags ItemGroup that represent the references of the project.
Doing that, save all files and close your VS and open again, now your project should load normally.
I hope that this informations help somebody to understand a bit more how the VS works and help solve the problems when necessary.