I created my project, I have been editing every day, and recently from scratch it failed to recognize an existing namespace

I created my project in Visual Studio 2013, I have been editing every day, and recently it has stopped recognizing an existing namespace.

The namespace appears in red with the message "The type or namespace 'Models' does not exist in namespace 'Project.Models'

However when running my project everything runs normally, even though this error appears in the Error List. It seems to be a Visual Studio bug, on another computer I opened the project and ran it normally!