how to use a code for login if user already exist below is my code but it's not working any one can help me

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;
using System.Data;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (IsPostBack)
        {
            SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["LEARNINGConnectionString"].ConnectionString);
            con.Open();
            string checkUser = "select count(*) from login where username='" + TextBox1.Text + "'";
            SqlCommand cod = new SqlCommand(checkUser, con);
            int temp = Convert.ToInt32(cod.ExecuteScalar().ToString());
            con.Close();
            if (temp == 1)
            {
                Response.Write("User already exist");
            }

        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        try
        {
            SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["LEARNINGConnectionString"].ConnectionString);
            con.Open();
            string InsertQuery = "insert into login (Username,password) values (@Username,@password)";
            SqlCommand cmd = new SqlCommand(InsertQuery, con);
            cmd.Parameters.AddWithValue("@Username", TextBox1.Text);
            cmd.Parameters.AddWithValue("@password", TextBox2.Text);
            cmd.ExecuteNonQuery();
            Response.Redirect("Admin.aspx");
            Response.Write(" registration sucessfully");
            con.Close();
            
            
            
        }
        catch (Exception ex)
        {
            Response.Write("error" + ex.ToString());

        }
    }
}