database connectivity in .net 2008

please try the following.

using System.Data.SqlClient;
using system.Data;

on click event of button please write

sqlconnection conn=new Sqlconnection("Your Connection String");
if(conn.state.equals(connectionstate.closed)
conn.open();
sqlcommand cmd=new sqlcommand("SELECT COUNT(ID) FROM table_name where Log_in='"+login_id+"' AND password='"+Password+"'",conn);
sqldataadapter da=new sqldataadapter(cmd);
if(cmd.executeNonQuery()>0)
response.write("<script>alert('valid user')</script>");
else
response.write("<script>alert('Invalid user')</script>");

i assumes that your field is primary key.replace the name of table with table_name of sqlcommand.