I have created a session var and I am using it in one of my pages. Sometimes the session var loses its value and nothing displays where I am using it. How am I losing this value?

Here is my code:

Sub Page_Load(Src As Object, E As EventArgs)
        Dim ConnectString As String
        MyConnect = New SqlConnection
        connectString = ConfigurationSettings.AppSettings("ConnectStr")
        MyConnect.ConnectionString = ConnectString
        Lbl_Biller.text = session("UserCode")
  If Not IsPostBack Then
      BindGrid()
      DropDown()
     End If

    End Sub