java script funcation for open panel

<script type="text/javascript">
        function ff() {
            document.getElementById("a").style.display = "block";
        }
        function ff2() {
            document.getElementById("a").style.display = "none";
        }
    </script>

 <div class="GridviewDiv" id="a" style="display: none">
contanet div that will be open
</div>

call
    <asp:DropDownList ID="DropDownList1" runat="server" onmouseover="javascript:ff()" >
    </asp:DropDownList>