Javascript Issues in Visual Studio 2013

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Javascript

The following error comes up in my javascript:

0x800a1391 - Microsoft JScript runtime error: '$' is undefined

this is my code:

<script src="Scripts/jquery-1.11.0.min.js" type="text/javascript"></script>
    <script type="text/javascript">
        $(document).ready(function () {

            $('#btnLogin').click(function () {

                //                    alert("Please fill in all fields");
                //                }

                //                else {

                var uname = $('#MainContent_tb_UserName').val();
                var pwd = $('#MainContent_tb_Password').val();
                if (uname != "" || pwd != "") {
                    $("#Login").css("display", "none");
                    $("#Loading").css("display", "block");
                    $.ajax({
                        type: "POST",
                        contentType: "application/json; charset=utf-8",
                        url: "Login.asmx/Log",
                        data: "{uname: '" + uname + "',pwd: '" + pwd + "'}",

                        dataType: "json",
                        success: function (result, success) {

                            var status = result.d;
                            if (status == "1") {
                                //                            $("#login").css("display", "none");
                                // response.redirect
                                window.location.href = 'Default.aspx';

                                //clear textboxes
                                $('ContentPlaceHolder1_tb_UserName').val("");
                                $('ContentPlaceHolder1_tb_Password').val("");

                            }
                            else {
                                alert("Login Failed");

                                $("#Login").css("display", "block");
                                $("#Loading").css("display", "none");
                            }

                        },
                        error: function (error) {




                        }

                    });

                }
            });
        });
        </script>

It was working perfectly but now it complains for some reason.

can anyone please help?