Sometimes, when you’re developing a visual web part, the designer file (with .g.cs extension) dissapears in front of your eyes.
After finding nothing in the internet, we’ve just discovered why it is happening:
You've to add the "Site URL" property in the Project properties.
That’s beacuse it is a web part for sharepoint, and Visual Studio must be able to acces to the site.
It’s a bug and we suppose that microsoft will resolve it but until then, we’ve to do this.