Add this script in your page:-
function SessionExpire()
{
var answer = confirm("Sorry, your session timed out. Click OK to redirect to Login page, Cancel to stay of same page")
if (answer){
window.location.href="./Login.aspx";
return true;
}
else{
return false;
}
}
and call from code behind:-
ScriptManager.RegisterStartupScript(this, GetType(), "SessionMsg", "SessionExpire();", true);