You haven't shared enough information, I'm helping you with my assumption.
<form>
<div class="line"></div>
<input type="checkbox" />
</form>
.line{
height:200px;
width:2px;
background-color:#f44;
display:none;
}
.show-line{
display:block;
}
$("input[type='checkbox']").change(function() {
$(".line").toggleClass("show-line");
});
Hope it helps.
-KR