.loader {
    display: none;
    position: fixed;
    z-index: 1100;
    top: 0;
    left: 0;
    height: 100%;
    width: 100%;
    background: rgba(200, 200, 200, .5) url(/Content/ajax-loader.gif) 50% 50% no-repeat;
}

body.loading {
    overflow: hidden;
}

    body.loading .loader {
        display: block;
    }