How Lock the process

not asking for permanent locking..
if we want to calculate next month calculation first we have to lock previous once right? for that i need query..
and also we want to lock the financial year while we move to next year process..
Whether we want to maintain one separate table for this process or how we have to proceed it further..

What i m doing is payroll project...
For that i am asking..
i hope now u wil come to know for what i am asking for...

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

namespace lockprocedure
{
public partial class _Default : System.Web.UI.Page
{

SqlDataReader reader;
SqlConnection conn;
SqlCommand cmd;
protected void Page_Load(object sender, EventArgs e)
{

}

protected void Button1_Click(object sender, EventArgs e)
{
string connectionString="Data Source=SNSWAMY\\SQLEXPRESS;Initial Catalog=payroll;Persist Security Info=True;User ID=******;Password=*********";
conn = new SqlConnection(connectionString);
string updateSql = "update lock_table set key_value=1 where sal_date='2012-02-25 00:00:00.000'";
cmd = new SqlCommand(updateSql, conn);


try
{
conn.Open();
cmd.ExecuteNonQuery();

}
catch (Exception)
{

}
finally
{
conn.Close();
}
MessageBox("Locked the previous month calculation");
}
private void MessageBox(string message)
{
if (!string.IsNullOrEmpty(message))
{
Response.Write("<script type=\'text/javascript' language=\'javascript\'>");
Response.Write("alert('" + message + "');");
Response.Write("</script>");
}
}


protected void Button2_Click(object sender, EventArgs e)
{
int key_value;
string connectionstring="Data Source=SNSWAMY\\SQLEXPRESS;Initial Catalog=payroll;Persist Security Info=True;User ID=******;Password=******";
conn = new SqlConnection(connectionstring);
conn.Open();
// string updateSql = "update salary set key_value=1 where process_datetime='2012-02-25 00:00:00.000'";
SqlCommand cmd = new SqlCommand(("select key_value from lock_table where sal_date='2012-02-25 00:00:00.000' "), conn);
// cmd = new SqlCommand(updateSql, conn);
cmd.Connection = conn;


reader = cmd.ExecuteReader();

//reader.Read();

/*if (reader.HasRows)
{
}*/
while (reader.Read())
{
// key_value = reader.GetOrdinal("key_value");
reader.Read();
reader["key_value"].ToString();
//key_value = reader.GetOrdinal("key_value");
//Console.WriteLine(reader.GetInt32(1));
/* if (key_value == 0)
{
MessageBox("Lock the previous Month Calculation");
}
else
{
MessageBox("Can proceed further");
}
reader.Close();*/

}

key_value = reader.GetOrdinal("key_value");
// int key_value =reader.GetInt32(reader.GetOrdinal("key_value"));

if (key_value == 0)
{
MessageBox("Lock the previous Month Calculation");
}
else
{
MessageBox("Can proceed further");
}
reader.Close();

}

}
}

-------------------------------------------------------------------------------
This is my code for lock procedure.. In this i am getting an error invalid attempt to read when no data is present. c# and also reader is not working properly because after i locked the previous month calculation again it is asking to lock