Connection string reference not set to an instance of an object

Question

This is a noob question but I have been stuck here for a few hours so I need to get passed this. I have this Windows App that performs a simple function. It makes a trip to the DB and checks if a specific record exists and if it does then perform some operation.
However it just doesn't want to read the connection string - it comes as null all the time.
I keep getting null every time I initialize my connection string. What could I be doing wrong. I only have one connection string named App.config.

Class File:

 private class ClassA 
 {
     private string myConnectionString = "";
     private SqlConnection mySQLConnection;
     private SqlCommand mySQLCommand;

     private int CheckIfSerialNumberExists(UInt64 ColumnToCheck)
     {
         int countResult = 0;
         myConnectionString = ConfigurationManager.ConnectionStrings["ConnectionStringName"].ConnectionString; //I get an object reference null here when the compiler executes this line. I have been using this structure for years and never got any issues
         using (mySQLConnection = new SqlConnection(myConnectionString))
         {
              string procName = "SELECT Count(*) ColumnName FROM Table WHERE ColumnName='" + ColumnToCheck + "'";
              mySQLCommand = new SqlCommand(procName, mySQLConnection);
              mySQLConnection.Open();
              countResult = (int)mySQLCommand.ExecuteScalar();
         }
         return countResult;
     }

     private void someFunc()
     {
        //Test value: 5
        if(CheckIfSerialNumberExists(5) > 0)
        {
           //Don't do anything
        }
        else
        {
           //Save to DB
        }
     }
 }

Config File:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <connectionStrings>   
    <add name="ConnectionStringName"
         connectionString="Data Source=ServerName;Initial Catalog=DatabaseName;Persist Security Info=True;User ID=**;Password=****"
         providerName="System.Data.SqlClient" />
  </connectionStrings>
<startup><supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5.1"/></startup>
</configuration> 

Answer 1

So here is the quick fix:

In my windows app I am referencing a class library project. I had only added the connection string in that project. I added it to my windows app project and there we go, all was working. Really feel like an idiot.