Session values goes null in MVC controller

See more:

C#3.5

MVC5

hi,
I am working with mvc. my code is working fine till last day.But now i am facing an strange problem. I have set a value in Session. Here is code

  public ActionResult RecordForEdit(string ID)
        {
     int Idd = Convert.ToInt32(ID);
            Session["SubuserId"] = Idd;            
            var model = new SubUserModel();
             model.UserName = obd_Sub.LoginId;                          
                model.Pwd = obd_Sub.Password;
                model.Status = obd_Sub.Status;
            return View("SubUserIndex", model);
}

then in view a method is called by Jquery .

public JsonResult GetAssigned_Suburb()
       {
           string UId ="";
           if (Session["SubuserId"] != null)
           {
              UId = Session["SubuserId"].ToString();
           }
           var Suburbs = odbe.ODB_GetSuburb_onload(UId);
           var jsonResult = Json(Suburbs.ToList(), JsonRequestBehavior.AllowGet);
           return jsonResult;
       }

in this method i get the value of

Session["SubuserId"]

, then again on a button click a method is called.

public JsonResult GetSuburb_ToAssign(string Pcode, string SubId)
        {
            string UId = Session["UserId"].ToString();
            var UsrId = from s in odbe.ODB_UserMain where s.LoginId == UId select s;
            string intId = UsrId.FirstOrDefault().Id.ToString();           
            string sub_Uid = "0";
            if (Session["SubuserId"] != null)
            {
                sub_Uid = Session["SubuserId"].ToString();
            }
            var Suburb = odbe.ODB_GetSubuserSuburb_Count(Pcode, intId, sub_Uid);
            var jsonResult = Json(Suburb.ToList(), JsonRequestBehavior.AllowGet);
            return jsonResult;
        }    

But in this method the method the value of

Session["SubuserId"]

is null. I not get how it is gone null.

Thanks

What I have tried:

I have debug and googled alot but nothing found