hey im getting the error of open please solve it for me

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;

public partial class SignUp : System.Web.UI.Page
{
    DateTime dt;
    SqlCommand cmd;
    SqlConnection cn;
    
    protected void Page_Load(object sender, EventArgs e)
    {
        cn = new SqlConnection(ConfigurationManager.AppSettings["cn"]);
        cn.Open();===>this was error it was giving
        
    }
    protected void btnRegister_Click(object sender, EventArgs e)
    {
        SqlDataReader dr;


    

    
        cmd = new SqlCommand("Select * From Registration where uname = '" + txt_uname.Text + "' or email = '" + txt_email.Text + "'", cn);
        dr = cmd.ExecuteReader();
        if (dr.Read())
        {
            lblerr.Text = "User Already Exists Try Again.";
        }
        else
        {
            dr.Close();
            string qu = "INSERT INTO Registration values ('" + txt_uname.Text + "','" + txt_pass.Text + "','" + txt_fname.Text + "','" + txt_mname.Text + "','" + txt_lname.Text + "','" + txt_address.Text + "','" + txt_email.Text + "','" + drp_gender.SelectedItem.Value + "','" + drp_country.SelectedItem.Value + "','" + txt_city.Text + "'," + txt_mobile.Text + ",'" + drp_sque.SelectedItem.Text + "','" + txt_ans.Text + "','')";
            cmd = new SqlCommand(qu);
            cmd.Connection = cn;
            cmd.ExecuteNonQuery();
            lblerr.Text = "User is Registered.";
            Response.Redirect("Login.aspx");
        }
    }
}