ASP.Net WebMethod: An object reference is required for the non-static field, method, or property 'System.Web.UI.Page.Session.get'

The following is not properly work.

//Javascript Code

<script type="text/javascript">
    $(function () {
        $("[id*=btnlogin]").bind("click", function () {
            var user = {};
            user.Username = $("[id*=txtUsername]").val();
            user.Password = $("[id*=txtPassword]").val();
            $.ajax({
                type: "GET",
                url: "Login.aspx/ValidateUser",
                data: '{user: ' + JSON.stringify(user) + '}',
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                success: function (response) {
                    alert(response.d);
                }
            });
            return false;
        });
    });
</script>

//HTML Code

<form id="form1" runat="server">
 <table border="0" cellpadding="0" cellspacing="0">
    <tr>
        <td>
            Username:
        </td>
        <td>
            <asp:TextBox ID="txtUsername" runat="server" Text="" />
        </td>
    </tr>
    <tr>
        <td>
            Password:
        </td>
        <td>
            <asp:TextBox ID="txtPassword" runat="server" TextMode="Password" />
        </td>
    </tr>
    <tr>
        <td>
        </td>
        <td>
            <asp:Button ID="btnlogin" Text="Log In" runat="server" />
        </td>
    </tr>
</table>
</form>

//C# Code

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


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

    [WebMethod]
    [ScriptMethod]
    public string ValidateUser(Users user)
    {
        int userId = 0;
        string constr = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;
        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand("Validate_Admin"))
            {
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.AddWithValue("@Username", user.Username);
                cmd.Parameters.AddWithValue("@Password", user.Password);
                cmd.Connection = con;
                con.Open();                
                userId = Convert.ToInt32(cmd.ExecuteScalar());
                con.Close();
            }
        }
        string IsValidate = string.Empty;
        switch (userId)
        {
            case -1:
                IsValidate = "Login Successfully..";       
                break;
            case -2:
                IsValidate = "Password is Incorrect..";
                break;
            case -3:
                IsValidate = "Username and password is incorrect..";
                break;
        }
        return IsValidate;     
    }    
}
public class Users
{
    public string Username { get; set; }
    public string Password { get; set; }
}

//Stored Procedure

ALTER PROCEDURE dbo.Validate_Admin
(
   @Username Varchar(40),
   @Password Varchar(40)
)
As
	if Exists
	(
	  Select AdminId From Admin where Username = @Username And Password  = @Password
	)
	SELECT -1 -- check for valid usrname and passwword

	if Exists
	(
	  Select AdminId From Admin where Username = @Username
	)
	SELECT -2 -- Username doesn't exist

SELECT -3 -- check for valid username