strange problem with a namespace

I copy/pasted some code from one webservice to another. 

this line of code now generated an error.

"Error 3 The name 'ConfigurationManager' does not exist in the current context C:\dev\webProjects\ExpressionBlend\TreeSite\usersWCF\Service1.cs 109 36 usersWCF

for this line of code.

string strConnection = ConfigurationManager.ConnectionStrings["db1"].ToString();
so I tried writing it out the long way, System.Configuration.ConfigurationManager and there was no ConfigurationManager listed as part of the library.  There were only a few choices, where before there were many.

 

So, I went back to my previously compiled without error code and removed t ...