from site master page ASP:Menu disappears when click on any button from any page

Hello 

I am using ASP: menu in my site.master page '

but when I clicked on any button or drop down from other page the manu is disappeared. 

can you please tell me why my menu is disappearing from the site.master page ?

My code is 

 <asp:Menu ID="MainMenu" runat="server" Orientation ="Horizontal"  CssClass="menu" EnableViewState="false" IncludeStyleBlock="false" >

        <Items > 

        <asp:MenuItem  Text ="Home" NavigateUrl="AllEMP.aspx" >

              </asp:MenuItem>

            <asp:MenuItem  Text ="Menu" NavigateUrl ="~/Default.aspx">

                   </asp:MenuItem>

  </asp:MenuItem>

        </Items>

        </asp:Menu>