code not working to check email already registered or not

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;

public partial class new_add_donor : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (IsPostBack)
        {
            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["mandConnectionString"].ConnectionString);
            conn.Open();
            string checkdonor = "select count(*) from mydatabase where email='"+TextBox4.Text+"'";
            SqlCommand com = new SqlCommand(checkdonor, conn);
            int temp = Convert.ToInt32(com.ExecuteScalar().ToString());
            if (temp == 1)
            {
                Response.Write("You Email ID is Already Registered Click on Forget Password if you have not Password");
            }

            conn.Close();
        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        try
        {
            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["mandConnectionString"].ConnectionString);
            conn.Open();
            string insertQuery = "insert into mydatabase (dob,name,father,gender,bgroup,distt,tehsil,cv,email,password,repassword,no1,no2,av_not,userdate,user_type) values (@dDate ,@dname ,@dfather ,@dgender ,@dbg ,@ddistt ,@dtehsil ,@dcv ,@demail ,@dpass ,@drepass ,@dno1 ,@dno2 ,@dav ,@ddate ,@duser1)";
            SqlCommand com = new SqlCommand(insertQuery, conn);

            com.Parameters.AddWithValue("@ddob", TextBox1.Text);
            com.Parameters.AddWithValue("@dname", TextBox2.Text);
            com.Parameters.AddWithValue("@dfather", TextBox3.Text);
            com.Parameters.AddWithValue("@dgender", DropDownList1.Text);
            com.Parameters.AddWithValue("@dbg", DropDownList2.Text);
            com.Parameters.AddWithValue("@ddistt", DropDownList3.Text);
            com.Parameters.AddWithValue("@dtehsil", DropDownList4.Text);
            com.Parameters.AddWithValue("@dcv", DropDownList5.Text);
            com.Parameters.AddWithValue("@demail", TextBox4.Text);
            com.Parameters.AddWithValue("@dpass", TextBox5.Text);
            com.Parameters.AddWithValue("@drepass", TextBox6.Text);
            com.Parameters.AddWithValue("@dno1", TextBox7.Text);
            com.Parameters.AddWithValue("@dno2", TextBox8.Text);
            com.Parameters.AddWithValue("@dav", DropDownList6.Text);
            com.Parameters.AddWithValue("@ddate", TextBox9.Text);
            com.Parameters.AddWithValue("@duser1", TextBox10.Text);
            com.ExecuteNonQuery();

            Response.Write("<script>alert('Dairy has been Saved Successfully.');</script>");

            conn.Close();
        }
        catch (Exception ex)
        {
            Response.Write("Error:" + ex.ToString());
        }
    }
}