Surprising Errors on system designed pages in VS 2012

I had a VS 2012 project with a target framework of .Net 2.0.  I have not been in it since last summer.  However, when I opened it and tried to run in Debug to deal with some applicationhost.config issues, suddenly I am getting errors for code that was put in place automatically by VS when the project was created.  Examples:
On the Site.Master page "'System.Guid' does not contain a definition for 'TryParse'"
In file OpenAuthProviders.ascx.cs, "The type or namespace name 'AspNet' does not exist in the namespace 'Microsoft' (are you missing an assembly reference?) "  Also in Manage.aspx.cs, AuthConfig.cs, and Register.aspx.cs .
Global.asax.cs, Contact.aspx.cs, Register.aspx.cs, Manage.aspx.cs, Login.aspx.cs, About.aspx.cs, AuthConfig.cs, BundleConfig.cs, Default.aspx.cs, and Site.Master.cs the error, "The type or namespace name 'Linq' does not exist in the namespace 'System' (are you missing an assembl ...