Null Error Reading Connection String

I am getting a null reference exception when trying to connect to the database via ConnectionCtrings["MyDB"].connectionstring.

This is in a using statement and is the same code I've used in many other projects, but it keeps telling me null reference and I don't know why.

The connection string named is named correctly in the web.config which is in the same project so I wouldn't expect there to be permissions issues.

What have I missed?

Edit: I have seen the suggested answers, these are solved by putting the string in the Web.Config which is where the connection string is.

Code: ConnectionString in Web.config

<connectionStrings>
    <add name="MyDB" connectionString="Data Source=192....; Initial Catalog=ProjectDb; Integrated Security=false; User Id=user; Password=password;" providerName="System.Data.SqlClient" />
</connectionStrings>

Function to access DB

public static Company RetrieveCompany(int id)
{
    var cmp = new Company();
    try
    {
        using (var con = new SqlConnection(ConfigurationManager.ConnectionStrings["MyDB"].ConnectionString))
        {
            con.Open();
            using (var cmd = new SqlCommand("RetreiveEmailProc", con))
            {
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.AddWithValue("@companyId", id);

                SqlDataReader sdr = cmd.ExecuteReader();
                while (sdr.Read())
                { // code omitted }
            }           

        }
    }
    catch(Exception ex)
    {

    }

    return cmp;
}