<add key="SiteTitle" value="Breaking News, World News, and Multimedia"/>
    <add key="SiteName" value="New York Times"/>
<add key="somekey" value="somevalue"/>

public string GetKeyThatHasMyValue (string Section, string ValueToFind)
{
    string Result = null;
    var Settings = (NameValueCollection)ConfigurationManager.GetSection (Section);
    foreach (string Key in Settings.Keys)
    {
        if (Settings[Key].Contains (ValueToFind))
        {
            Result = Key;
            break;
        }
    }

    return Result;
 }

using System.Collections.Specialized;

NameValueCollection lstKeyValues;
lstKeyValues = (NameValueCollection) ConfigurationManager.GetSection("YourSectionHere");