change/forgot password with a hashed password possible? - asp.net

Question

i have implemented my user passwords to be hashed. And what i want is to implement a forgot/change password. However i am not able to convert the hashed password to the original password and that gives me a failure to do the forgot/change password feature. Here is my code from my registration page:
cmd.Parameters.AddWithValue("#Password", BusinessLayer.ShoppingCart.CreateSHAHash (txtPW.Text));
Here is my creathash code:
public static string CreateSHAHash(string Phrase)
{
SHA512Managed HashTool = new SHA512Managed();
Byte[] PhraseAsByte = System.Text.Encoding.UTF8.GetBytes(string.Concat(Phrase));
Byte[] EncryptedBytes = HashTool.ComputeHash(PhraseAsByte);
HashTool.Clear();
return Convert.ToBase64String(EncryptedBytes);
}
and my changepassword page:
protected void btn_update_Click(object sender, EventArgs e)
{
SqlConnection con = new SqlConnection(conn);
con.Open();
str = "select * from UserData ";
com = new SqlCommand(str, con);
SqlDataReader reader = com.ExecuteReader();
while (reader.Read())
{
if (txt_cpassword.Text == reader["Password"].ToString())
{
up = 1;
}
}
reader.Close();
con.Close();
if (up == 1)
{
con.Open();
str = "update UserData set Password=#Password where UserName='" + Session["New"].ToString() + "'";
com = new SqlCommand(str, con);
com.Parameters.Add(new SqlParameter("#Password", SqlDbType.VarChar, 500));
com.Parameters["#Password"].Value = (txt_npassword.Text);
com.ExecuteNonQuery();
con.Close();
lbl_msg.Text = "Password changed Successfully";
}
else
{
lbl_msg.Text = "Please enter correct Current password";
}
}
What i want to do is to be able to convert my hashed password to the original password for it to be changed. Any tricks? or is it possible though?