You are trying to use Request.Cookies.Value without checking it the cookie exists.
This is the correct check:
string roll = string.Empty;
if (Request.Cookies["StudentCookies"] != null) {
roll = Request.Cookies["StudentCookies"].Value;
}
' if cookie wasn't present or the value is empty, redirect
if ( roll == string.empty)
Response.Redirect("~/LoginPage.aspx");
Good luck.