if (!string.IsNullOrEmpty(txtUser.Text))
            {
                string constring = ConfigurationManager.ConnectionStrings["ConnData"].ConnectionString;
                SqlConnection con = new SqlConnection(constring);

                SqlCommand cmd = con.CreateCommand();
                cmd.CommandType = CommandType.Text;
                cmd.CommandText = "select Username from UserLogin where Username ='" + txtUser.Text + "'";

                DataTable dt = new DataTable();
                SqlDataAdapter da = new SqlDataAdapter(cmd);
                da.Fill(dt);

                if (dt.Rows.Count > 0)
                {
                    txtUser.Clear();
                    PasswordChange pc = new PasswordChange();
                    pc.Show();
                    this.Hide();
                    return;
                }
                else
                {
                    MessageBox.Show("Your username incorrect");
                }
            }
            else
            {
                MessageBox.Show("Please enter your username");
            }
        }