using System;
    using System.Data;
    using System.Linq;
    using System.Web;
    using System.Web.UI;
    using System.Web.UI.WebControls;
    using System.Collections.Generic;
    using System.Data.SqlClient;
    using System.Configuration;

    namespace WebApplication21

    {
        public partial class WebForm1 : System.Web.UI.Page
        {
            protected void Page_Load(object sender, EventArgs e)
            {

            }
           protected void Button1_Click(object sender, EventArgs e)
            {

             SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["dbconnection"].ConnectionString);
            con.Open();
         SqlCommand cmd = new SqlCommand("select * from 'user_insert' where username = @username and password = @password,con");
              cmd.Parameters.AddWithValue("@username", TextBox1.Text);
              cmd.Parameters.AddWithValue("@password", TextBox2.Text);
          SqlDataAdapter da = new SqlDataAdapter(cmd);
          DataTable dt = new DataTable();
          da.Fill(dt);

          if (dt.Rows.Count > 0)
          {
              Response.Redirect("Welcom.aspx");
          }
          else
          {
              Response.Redirect("Error.aspx");
          }

    }

<connectionStrings>
<add name="dbconnection" connectionString="Data Source=Ali-PC;Initial Catalog=LogIn;Integrated Security=True"/>
</connectionStrings>

SqlCommand cmd = new SqlCommand
    (@"select * from 'your table' where username = @username and password = @password",con);
cmd.Parameters.AddWithValue("@username", TextBox1.Text);
          cmd.Parameters.AddWithValue("@password", TextBox2.Text);

          DataTable dt = new DataTable();
          dt.Load(cmd.ExecuteReader());