How to decode Hash password to string in SHA 256

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;
using System.Net.Mail;
using System.Text;
namespace Ordering_System
{
    public partial class ForgetPassword : System.Web.UI.Page
    {
        string cs = Global.CS;
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void btn_Submit_Click(object sender, EventArgs e)
        {
            string email = TextBox1.Text;
            string sql = "SELECT Username,Hash,MemEmail FROM Member";
            SqlConnection con = new SqlConnection(cs);
            SqlCommand cmd = new SqlCommand(sql, con);

            con.Open();
            if (con.State == ConnectionState.Open)
            {
                SqlDataReader dtr = cmd.ExecuteReader();

                while (dtr.Read())
                {
                    if (dtr[2].ToString().Equals(TextBox1.Text))
                    {
                        MailMessage mail = new MailMessage();
                        mail.To.Add(dtr[2].ToString());
                        mail.From = new MailAddress("redactedEmail@xxx.yyy");
                        mail.Subject = "Your userId and Password";
                        mail.Body = "<h2>DailyDemand<br />Username and Password Recovery</h2><br />Username :" + dtr[0].ToString() + "<br />" + "Password :" + dtr[1].ToString() + "";
                        mail.IsBodyHtml = true;
                        SmtpClient client = new SmtpClient();
                        client.Port = 587;
                        client.EnableSsl = true;
                        client.Timeout = 10000;
                        client.Host = "smtp.xxx.yyy";
                        client.DeliveryMethod = SmtpDeliveryMethod.Network;
                        client.UseDefaultCredentials = false;
                        client.Credentials = new System.Net.NetworkCredential("redactedEmail@xxx.yyy", "PASSWORD");
                        client.EnableSsl = true;
                        client.Send(mail);
                        Label1.Text = "Your Password already sent to your Email, Please Check ";


                        break;
                    }
                    else
                    {
                        Label1.Text = "Email is not Valid! Please Try Again! <br /> <a href="MemReg.aspx" style="color:Blue">Sign Up</a><br />";
                    }

                }
            }
        }
    }
}