in client side use javascript :
document.getElementById("BodyMainContent").style.visibility="hidden";
in server side using c# first replace panel instead of div:
1- if you need ajax you should first have a script manager and define a trigger for mouseclick event then add runat="server" to panel tag then use this code :
Panel1.visible=False
2- if you dnt need ajax so just add same code :
Panel1.visible=False