Visual Studio 2012 Tests Windows Phone App suddenly fail due to missing reference

Visual Studio 2012 Tests Windows Phone App suddenly fail due to missing
reference

I am developing an app for Windows Phone, and also have a Test project for
creating and running tests. This was working fine, but suddenly I am
getting an error that the reference System.Runtime.Serialization cannot be
found.
Specifically the error is:
System.IO.FileNotFoundException: Could not load file or assembly
'System.Runtime.Serialization, Version=2.0.5.0, Culture=neutral,
PublicKeyToken=7cec85d7bea7798e' or one of its dependencies.
First of all, shouldn't it be 4.0 not 2.0.5?
I attempted to add a reference to the 4.0 version of
System.Runtime.Serialization, but now the error is this:
System.IO.FileLoadException: Could not load file or assembly
'System.Runtime.Serialization, Version=4.0.0.0, Culture=neutral,
PublicKeyToken=7cec85d7bea7798e' or one of its dependencies. The located
assembly's manifest definition does not match the assembly reference.
I looked all over the projects to find where this reference (2.0.5) is
defined so I can update it (not sure how it ever got changed) but have had
no luck.
I am using JSON.NET if that matters (5.0.6)
If it is supposed to be 2.0.5 is there any way the project could have been
accidentally or automatically changed to be 4.0?
this was working before, so SOMETHING changed, and it certainly wasn't
deliberately done by me...