Login - check database if user exists

int cntUser = DBHelper.DB.ExecuteScalar("SELECT COUNT (*) FROM [signin] WHERE username='" + txtusername.Text + "' AND password='" + txtpassword.Text + "'");
if (cntUser > 0)
{
    frmMain main = new frmMain();
    main.Show();
}
else
{
    MessageBox.Show("Invalid User Name or Password");
}

using System;
using System.Collections.Generic;
using System.Text;
using System.Data;
using System.IO;
using System.Windows.Forms;
using System.Data.OleDb;
namespace DBHelper
{
    public sealed class DB
    {
        //You need to verify following connection string
        const string DBConnection = "Server=NGENIOUS-WSSDEV\SQLEXPRESS;Database=MIC;Trusted_Connection= True";
        private DB()
        {
        }
        public static OleDbDataReader getDataReader(string strsql)
        {
            try
            {
                OleDbConnection con = new OleDbConnection();
                con = getConnection();
                OleDbCommand cmd = new OleDbCommand(strsql, con);
                OleDbDataReader dr = cmd.ExecuteReader();
                return dr;
            }
            catch (Exception ex)
            {
                MessageBox.Show("Sorry! for inconvenience.\nPlease report following error to imdadhusen.sunasara@gmail.com, If you can see more than once.\n" + ex.Message, "Database Error", MessageBoxButtons.OK);
            }
            return null;
        }
        public static DataTable getDataTable(string strsql)
        {
            try
            {
                OleDbConnection con = new OleDbConnection();
                con = getConnection();
                OleDbDataAdapter da = new OleDbDataAdapter(strsql, con);
                DataSet ds = new DataSet();
                da.Fill(ds);
                //closeConnection(con);
                return ds.Tables[0];
            }
            catch (Exception ex)
            {
                MessageBox.Show("Sorry! for inconvenience.\nPlease report following error to imdadhusen.sunasara@gmail.com, If you can see more than once.\n" + ex.Message, "Database Error", MessageBoxButtons.OK);
            }
            return null;
        }
        public static int ExecuteNonQuery(string strsql)
        {
            int isSuccess = 0;
            try
            {
                OleDbConnection con = new OleDbConnection();
                con = getConnection();
                OleDbCommand cmd = con.CreateCommand();
                cmd.CommandText = strsql;
                isSuccess = cmd.ExecuteNonQuery();
                closeConnection(con);
            }
            catch (Exception ex)
            {
                MessageBox.Show("Sorry! for inconvenience.\nPlease report following error to imdadhusen.sunasara@gmail.com, If you can see more than once.\n" + ex.Message, "Database Error", MessageBoxButtons.OK);
            }
            return isSuccess;
        }
        public static int ExecuteScalar(string strsql)
        {
            int isSuccess = 0;
            OleDbConnection con = new OleDbConnection();
            con = getConnection();
            OleDbCommand cmd = con.CreateCommand();
            cmd.CommandText = strsql;
            try
            {
                isSuccess = Convert.ToInt32(cmd.ExecuteScalar());
            }
            catch
            {
                isSuccess = 0;
            }
            closeConnection(con);
            return isSuccess;
        }
        public static OleDbConnection getConnection()
        {
            OleDbConnection con = new OleDbConnection();
            try
            {
                string MDBFilePath = Environment.GetFolderPath(Environment.SpecialFolder.System) + "\\SBSysDB.mdb";
                string strcon = DBConnection.Replace("{0}", MDBFilePath);
                con = new OleDbConnection(strcon);
                if (con.State != ConnectionState.Open) con.Open();
            }
            catch (Exception ex)
            {
                //MessageBox.Show(ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                MessageBox.Show("Sorry! for inconvenience.\nPlease report following error to imdadhusen.sunasara@gmail.com, If you can see more than once.\n" + ex.Message, "Database Error", MessageBoxButtons.OK);
            }
            return con;
        }
        public static void closeConnection(OleDbConnection con)
        {
            if (con.State == ConnectionState.Open) con.Close();
        }
    }
}