Registration of login

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C#

ASP.NET

SQL

Hello everyone... i have a login form which collects the password from the database... the sql database is called NumberPlate and the table is called LoginUser... i have also created a very simple registration form with only ENTER OLD PASSWORD and ENTER NEW PASSWORD...
what i want to do is from the registration form.. it should delete the old password from the LoginUser table and put a new password which i have just entered...

heres the problem...
the database is not deleting the old password and not adding the new one...

more details
1) i only having one user
2) i already have a password but i want to replace it
all i have is a 
a) login screen which contains a password text field
b) registration which only has 2 textfields of old password and new password
c) and a form which has nothing to do with users information

The user is only meant to log in and thats it... the user has no personel information in the database

iam really sry but my code is a little messed up... any help would be appreciated... if you would like more details pls do tell me... i would also appreciate if you correct the code for me...

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Data.Sql;
using System.Data.SqlClient;

namespace Number_Plate
{
    public partial class Login : Form
    {
        public Login()
        {
            InitializeComponent();
        }

        SqlDataAdapter da = new SqlDataAdapter();
        SqlConnection sc = new SqlConnection("Data Source=MAAZA-PC;Initial Catalog=NumberPlates;Integrated Security=True");

        private void BtnLogin_Click(object sender, EventArgs e)
        {
            try
            {
                sc.Open();
            }
            catch (Exception)
            {
                MessageBox.Show("Did not connect");
            }
            SqlCommand cmd = new SqlCommand("SELECT * FROM LoginUser", sc);
            cmd.Connection = sc;
            SqlDataReader reader = null;
            reader = cmd.ExecuteReader();
            while (reader.Read())
            {
                if (TxtOldPassword.Text == (reader["Password"].ToString()) && TxtPassword.Text.ToString().Length > 0)
                {
                    <pre>int x;

                    da.UpdateCommand = new SqlCommand("UPDATE LoginUser SET password=@password", sc);
                    da.UpdateCommand.Parameters.Add("@Password", SqlDbType.VarChar).Value = TxtPassword.Text;
                    sc.Open();
                    x = da.UpdateCommand.ExecuteNonQuery();
                    sc.Close();

                    if (x >= 1)
                    MessageBox.Show("Password reset"

);
}
else
{
MessageBox.Show("Pawwsord not reset");
}
}
}
}
}

now the error is coming as "connection was not closed. connection state still opened"

but what is suppose to happen is tht the LoginUser table should be updated and password is suppose to be the one in TxtPassword

thanks both of u...