Name 'Session' is not declared

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ASP.NET

Hello Friends,
When I load my ASP.Net 2008 project, I often get errors in the VB code behind file such as the one posted above. Sometimes, it sales a textbox object or label is not defined, even if it is and as I result I cannot load my project. My code looks like this one:

        On Error GoTo Err_Page_Load
        Dim strMsg As String

        If Session("GLSys") = True Then
            mnuMainMenu.Items(1).Enabled = True
        End If
        lblUserName.Text = Session("UserName")
        lblPeriodNumb.Text = Session("PeriodNumb")
        strMsg = CStr(session("StartDate")) & "-" & CStr(session("EndDate"))
        'lblDateRange.Text = Session("StarDate").ToString
        'lblDateRange.Text = lblDateRange.Text & "-" & Session("EndDate").ToString
        lblDateRange.Text = strMsg
        lblAcctYear.Text = Session("CurrYear").ToString & "-" & Session("NextYear").ToString
        lblLocCurrCode.Text = Session("LocCurrCode")
        lblDateClock.Text = DateTime.Today.ToString
        lblCoyName.Text = Session("CoyName") '& "->" & Session("BranchCode")
        lblCopyright.Text = Session("ProgTitle")
        imgCoyLogo.ImageUrl = "~/Images/CoyLogo.jpg"
Exit_Page_Load:
        Exit Sub

Err_Page_Load:
        lblStatus.Text = lblStatus.Text & "Error No : " & Err.Number.ToString & "->" & Err.Description & " has occurred!" & Chr(13)
        lblStatus.Visible = True
        Resume Next

Could someone please help! How can I solve this problem and why do I get these types of messages.

Thanks to all!
Sylvester