div遮罩层，显示不出来,该怎么解决

<div id="cover" style="position:relative;left: 0;top: 0;height:0;width:0;display:none;background:gray;z-index: 1000;filter:alpha(opacity=30);"></div>    <div id="cover_msg" style="display:none;position:absolute;left:0;top:0;height:100;width:200;background:white;z-index: 1001;">正在导入，请稍后...</div>

var $cover = $J("#cover");var $cover_msg = $J("#cover_msg");function basedataMgtCheckBasedata() {    divShow("正在导入及计算数据问题，请稍后......");    $J.post("dataReportMgt_dataReportMgtSaveAdd.action", {        "reportTaskVO.id" : $J('#reportTaskVOId').val(),        "ReportTaskFileVOStrings":$J("#fileIds").val(),        "reportTaskStatus" : 1    }, function() {        divHide();        alert("导入完成");        $J('#mytabs').tabs('add', {            title : '2.数据核查',            content : "<div id='problemData' style='width: 100%; height: 700px;'></div>",            closable : false        });    });}function divShow(msg){    $cover.width($J("body").width());    $cover.height($J("body").height());    $cover_msg.css("left",$J("body").width()/2-100);    $cover_msg.css("top",$J("body").height()/2-50);    $cover_msg.text(msg);        $cover.show();        $cover_msg.show();}function divHide(){    if($cover.is(":visible")){        $cover.hide();    }    if($cover_msg.is(":visible")){        $cover_msg.hide();    }}