make sure that jquery-ui.css file is added or not
<link rel="stylesheet" type="text/css" href="css/jquery-ui.css">or
<script type="text/javascript" src="js/jquery1.11.1-min.js"></script>
<script type="text/javascript" src="js/jquery-ui.js"></script>or<style type="text/css">
.ui-dialog{
z-index:103;
}
.ui-widget-overlay{
z-index: 102;
}
</style>or
add the following script
$("#mydialog").dialog({ modal: true, autoOpen: false,
open: function (event, ui) {
$('.ui-dialog').css('z-index', 103);
$('.ui-widget-overlay').css('z-index', 102);
}
});