So after upgrading to Visual Studio 2013 update 1 and upgrading my web project to MVC 5.1 I could no longer load or build the project. I keep getting an error from MSBuild stating that the TargetPlatformVersion variable evaluated to an empty string. Naturally this caused some initial headache.
After comparing the project file with a clean project and finding no major differences I moved on to looking at the solution file. It turns out the VisualStudioVersion had been bumped to “12.0.30110.0” up from the previous “12.0.21005.1”. After updating this to the newest version the project successfully loaded again.
So if you experience this issue update your solution files visual studio version and reload the solution.