blockUI（）没有阻塞

Question

function load()  
{  
    var una=$('#un').val();  
    $.ajax({
        url:'student/checkuser.php?na='+una,
        type:'GET',
        success:function(data){
            if(data==1){alert('it works fine');}
            else{alert('re enter');}
        }
    });
}
$(document).ajaxStop($.unblockUI);
$(document).ready(function(){
    $('#bt1').click(function(){
        $.blockUI();
        load();
    });
});

blockUI无效 函数不起作用我已经使用了js文件jquery和jquery.blockUI.js