myReader =  SelectCommand.ExecuteReader();

MySqlConnection conn = new MySqlConnection("secretstring")
        MySqlCommand SelectCommand = new MySqlCommand("SELECT * FROM userspassword where'" + this.loginuser.Text + "'and password'" + this.passworduser.Text, conn);
        MySqlDataReader myReader;
        conn.Open();
        //myReader = SelectCommand.ExecuteReader();
        myReader =  SelectCommand.ExecuteReader();


        int count = 0;
        while (myReader.Read())
        {
            count = count + 1;
        }
        if (count == 1)
        {
            MessageBox.Show("Correct");
            Form2 pannel = new Form2();
             pannel.Show();
             Hide();

        }
        else if (count > 1)
        {
            MessageBox.Show("More then 1 user logged in");
        }
        else
            MessageBox.Show("Incorrect password or username");
        conn.Close();

MySqlCommand SelectCommand = new MySqlCommand("SELECT * FROM userspassword where " +
        "username = '" + this.loginuser.Text + "' and password = '" +
         this.passworduser.Text +"'" , conn);

MySqlCommand SelectCommand = new MySqlCommand("SELECT * FROM userspassword where " +
        "username = @uname and password = @pwd", conn);
 SelectCommand.Parameters.AddWithValue("@uname",this.loginuser.Text);
 SelectCommand.Parameters.AddWithValue("@pwd",this.passworduser.Text);
 myReader =  SelectCommand.ExecuteReader();

"SELECT * FROM userspassword where'" + this.loginuser.Text
                                   ^^^^^^^ Column name is missing..

MySqlCommand SelectCommand = new MySqlCommand("SELECT * FROM userspassword where " + "YourColumnName = '" + this.loginuser.Text + "' and password '" + this.passworduser.Text +"'" , conn);

MySqlCommand SelectCommand =
    new MySqlCommand("SELECT * FROM userspassword where username='"
                     + this.loginuser.Text + "' and password='"
                     + this.passworduser.Text +"'", conn);