Session disappears unexpectedly - asp.net

Question

The site is a buy/sell site and the page the code comes from is the "add product" page.
The problem is that the session("change") becomes nothing by some reason, I can't find any errors. The payment.aspx have a button that sends me back to the page with a session("change").
The reason I see the problem is that when I try to edit something the category gets restetted to the first in the list. and when I debug I see that the session is nothing, though it should be something
Heres the code:
Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles BtnSubmit.Click
If Not stats > 0 Then
If Session("change") IsNot Nothing Then
Dim dc As New DataClassesDataContext
Dim getP = From prod In dc.Products _
Where prod.ProductID = CInt(Session("change")) _
Select prod
If getP.Any Then
If rdbSell.Checked = True Then
getP.FirstOrDefault.BuySell = True
Else
getP.FirstOrDefault.BuySell = False
End If
If ddlSubSubcat.SelectedValue IsNot String.Empty Then
getP.FirstOrDefault.CategoryID = CInt(ddlSubSubcat.SelectedValue)
Else
getP.FirstOrDefault.CategoryID = CInt(ddlSubCat.SelectedValue)
End If
getP.FirstOrDefault.Content = txtContent.Text.Replace(Environment.NewLine, "<br />")
getP.FirstOrDefault.CountyID = CInt(ddlCounty.SelectedValue)
getP.FirstOrDefault.E_mail = txtEmail.Text
getP.FirstOrDefault.Date = DateTime.Now
getP.FirstOrDefault.Active = 0
getP.FirstOrDefault.Alias = txtAlias.Text.Replace("'", "''")
getP.FirstOrDefault.ShowEmail = 0
Dim PreID As Integer = getP.FirstOrDefault.ProductID
If chkShowEmail.Checked = True Then
getP.FirstOrDefault.ShowEmail = 1
Else
getP.FirstOrDefault.ShowEmail = 0
End If
If chkShowPhone.Checked = True Then
getP.FirstOrDefault.ShowPhone = 1
Else
getP.FirstOrDefault.ShowPhone = 0
End If
getP.FirstOrDefault.Headline = txtHeadline.Text
getP.FirstOrDefault.Password = txtPassword.Text
getP.FirstOrDefault.Phone = txtPhone.Text
getP.FirstOrDefault.Price = txtPrice.Text
If chkUnknown.Checked = True Then
getP.FirstOrDefault.YearModel = String.Empty
Else
getP.FirstOrDefault.YearModel = ddlYear.SelectedValue
End If
For Each item In libPictures.Items
Dim i As String = item.ToString
Dim imagecheck = From img In dc.Pictures _
Where img.Name = i And img.ProductID = CInt(Session("change")) _
Select img
If imagecheck.Any Then
Else
Dim img As New Picture
img.Name = item.ToString
img.ProductID = CInt(Session("change"))
dc.Pictures.InsertOnSubmit(img)
dc.SubmitChanges()
End If
Next
dc.SubmitChanges()
Session.Remove("change")
Response.Redirect("~/precheck.aspx?id=" + PreID.ToString)
End If
Else
Dim dc As New DataClassesDataContext
Dim prod As New Product
If rdbSell.Checked = True Then
prod.BuySell = True
Else
prod.BuySell = False
End If
If ddlSubSubcat.DataValueField IsNot String.Empty Then
prod.CategoryID = CInt(ddlSubSubcat.SelectedValue)
Else
prod.CategoryID = CInt(ddlSubCat.SelectedValue)
End If
prod.Content = txtContent.Text.Replace(Environment.NewLine, "<br />")
prod.CountyID = CInt(ddlCounty.SelectedValue)
prod.E_mail = txtEmail.Text
prod.Date = DateTime.Now
prod.Active = 0
prod.Alias = txtAlias.Text.Replace("'", "''")
prod.ShowEmail = 0
If chkShowEmail.Checked = True Then
prod.ShowEmail = 1
Else
prod.ShowEmail = 0
End If
If chkShowPhone.Checked = True Then
prod.ShowPhone = 1
Else
prod.ShowPhone = 0
End If
prod.Headline = txtHeadline.Text
prod.Password = txtPassword.Text
prod.Phone = txtPhone.Text
prod.Price = txtPrice.Text
If chkUnknown.Checked = True Then
prod.YearModel = String.Empty
Else
prod.YearModel = ddlYear.SelectedValue
End If
dc.Products.InsertOnSubmit(prod)
dc.SubmitChanges()
Dim PreID As Integer = prod.ProductID
For Each item In libPictures.Items
Dim img As New Picture
img.Name = item.ToString
img.ProductID = prod.ProductID
dc.Pictures.InsertOnSubmit(img)
dc.SubmitChanges()
Next
Session.Remove("change")
Response.Redirect("./precheck.aspx?id=" + PreID.ToString, False)
End If
End If
stats = 0
'Catch ex As Exception
'End Try
End Sub

Answer 1

It depends upon how the application is managing session state. If your session state is managed InProc then if the application pool is recycled then all your session information will be lost. If that is happening then it could be a good option to store session state in SQL Server which will persist between app pool recycling.
More info:
ASP.NET Session State Overview
ASP.NET State Management Recommendations