Problem with website login

public class DataConnector
    {
        protected OleDbDataAdapter DataAdapter1 = new OleDbDataAdapter();
        public string ErrorMessage = "";
        public DataConnector(string ConnectionString)
        {
            OleDbConnection Connection1 = new OleDbConnection(ConnectionString);
            this.DataAdapter1.SelectCommand = new OleDbCommand("", Connection1);
            this.DataAdapter1.InsertCommand = new OleDbCommand("", Connection1);
        }
        public DataTable DataSelect(string query)
        {
            DataTable dt = new DataTable();
            try
            {
                DataAdapter1.SelectCommand.CommandText = query;
                DataAdapter1.SelectCommand.Connection.Open();
                DataAdapter1.Fill(dt);
                DataAdapter1.SelectCommand.Connection.Close();
                ErrorMessage = "";
            }
            catch(Exception err)
            {
                ErrorMessage = err.Message;
                DataAdapter1.SelectCommand.Connection.Close();
            }
            return dt;
        }
        public int DataInsert(string query)
        {
            int Result = 0;
            try
            {
                DataAdapter1.InsertCommand.CommandText = query;
                DataAdapter1.InsertCommand.Connection.Open();
                Result = DataAdapter1.InsertCommand.ExecuteNonQuery();
                DataAdapter1.InsertCommand.Connection.Close();
                ErrorMessage = "";
                return Result;
            }
            catch (Exception err)
            {
                ErrorMessage = err.Message;
                DataAdapter1.InsertCommand.Connection.Close();
                return 0;
            }
            
        }
        public int DataUpdate(string query)
        {
            return DataInsert(query);
        }
        public int DataDelete(string query)
        {
            return DataInsert(query);
        }
    }

public partial class Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void btnLogin_Click(object sender, EventArgs e)
        {

            DataLayer.DataConnector dat = new DataLayer.DataConnector("Provider=Microsoft.ACE.OLEDB.12.O;"+"Data Source='"+Server.MapPath("site_database.accdb")+"'; Persist Security Info=False;");
            DataTable dt = dat.DataSelect("select UserID from tbl_login where Username = '" + txtUsername.Text + "' and Password = '"+ txtPassword.Text +"' ");
            if (dt.Rows.Count > 0)
            {
                Response.Redirect("members_area.aspx");
            }
            else
                lblerror.Text = "Login failed";
            
        }
    }