.turn {

    position: absolute;

    width: 100%;

    height: 100%;

    left:;

    top:;

    background: url(../images.png) center center no-repeat #000;

    background-size: contain;

    display: none;

    z-index:;

}

@media ( orientation: landscape ) {

    .turn {

        display: block;

    }

}

<!--如果是横屏或电脑打开，提示它换成手机竖屏。-->

<div class="turn"></div>