css display=none 后 然后display=block 就无效了

要得可是这样的效果?
<div id="divBody" name="divBody" style="display:block"> 
      <div id="MessageBorld"> 
          <div id="content" class="content"> 
            <div id="title" class="title" onclick="ShowContent('plot');"> 
                Guest Book <a onClick="CloseContent(this)" href="#">[Close] </a> 
            </div> 
            <div id="plot" class="plot" > 
                <span style="font-size:36px; text-align:center;font-weight: bolder;">Building...... </span> 
                <h1>We holp you can give our some idea. thans </h1> 
            </div> 
          </div> 
      </div> 
</div> 
<script>
function ShowContent(obj) 
{ 
    var cobj=document.getElementById(obj);
    if(cobj.tagName == "DIV" )
    {
        if(cobj.style.display=="none")
            cobj.style.display="block";
        else
            cobj.style.display="none";
    }
} 
</script>