DataTable dt = (DataTable)Session["name"]; unable to cast

Hi dipti prasad,
it's simply means that Session["postData"] does not contains proper dataTable format. so you are getting this error. check Session["postData"] contains proper dataTable or not. 
if Session["postData"] contains only string then u need to add that string inside DataTable rows 
by using following code 
DataTable dt = new DataTable("PostData");
DataColumn colPData = new DataColumn("colPData",typeof(System.String));
dt.Rows.Add(Session["postData"].Tostring());