Page Index Changed. GridView data showing is lost

I have a GridView that shows particular data list or reports when clicking Generate button.

GridView Records

When I'm moving to the next page of the GridView or when I click the next page, the GridView binding becomes empty and it shows no GridView list.

Page Index Changed

Here's my code in .aspx

<asp:GridView ID="GridView1"
    runat="server"
    CellPadding="4"
    HeaderStyle-ForeColor="White"
    ForeColor="#333333"
    GridLines="None"
    CssClass="mGrid"
    PagerStyle-CssClass="pgr"
    AlternatingRowStyle-CssClass="alt"
    RowStyle-CssClass="row"
    AllowSorting="True"
    AllowPaging="True"
    PageSize="25" Visible="false" OnRowDataBound="GridView1_RowDataBound"
    EnableViewState="false">
    <EmptyDataTemplate>
        No record found.
    </EmptyDataTemplate>
</asp:GridView>

Here's my code for Page_Load

   Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
            userid = IIf(SessionHandler.UserID = "", User.Identity.Name.ToUpper, SessionHandler.UserID)

            V3Substance.Attributes.Add("onfocus", "javascript:if (this.value=='Substance Cas Number') { this.value = ''; }")
            V3Substance.Attributes.Add("onblur", "javascript:if (this.value=='') { this.value = 'Substance Cas Number'; }")

            If Not Page.IsPostBack Then
                V1Division.DataSource = GetDivision()
                V1Division.DataBind()

                V1BR.Items.Clear()
                V1BR.DataSource = GetBusinessRule(V1Division.Text)
                V1BR.DataBind()

                MultiView1.ActiveViewIndex = 0
                Panel1.DefaultButton = "V1Generate"
            End If
        End Sub

Here's my code for GridView1_PageIndexChanging

Protected Sub GridView1_PageIndexChanging(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewPageEventArgs) Handles GridView1.PageIndexChanging
    Dim dt As DataTable = TryCast(GridView1.DataSource, DataTable)
    GridView1.DataSource = SortTable(dt)

    GridView1.PageIndex = e.NewPageIndex
    GridView1.DataBind()
End Sub

And here's my code for BindGridView

 Private Sub BindGridview(ByVal query As String, ByVal countquery As String)
            ViewState.Add("Query", query)
            ViewState.Add("CountQuery", countquery)

            Dim count As Integer = 0

            If Description.Text = "1" Then
                ValidateFamily(query)
            Else
                DataSource = dbHelper.GetRecordDT(query)
            End If

            count = DataSource.Rows.Count

            'GridView1.DataSource = Nothing
            'GridView1.DataBind()
            'GridView1.PageIndex = 0

            GridView1.DataSource = DataSource
            GridView1.Visible = True
            GridView1.DataBind()

            ExportToExcel.Visible = IIf(count < 1, False, True)
            Panel1.Visible = IIf(count < 1, False, True)
            SetTotalResult(count)
        End Sub

I hope you can help me with this. I can't seem to know what could be the problem. I tried different approach to this and find a solutions from the internet but it didn't help. Thanks a lot.

You should be using

GridView1.DataSource = DataSource

It's unclear from your snippet what DataSource is, but you use it in the BindGridview method also.

Start using different names for variables, naming them exactly as properties is very confusing and will lead to errors.

Update

Dim source As SqlDataSource

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)       
    source = New SqlDataSource
    source.ID = "mySource"
    source.SelectCommand = "select * from table"
    source.ConnectionString = connStr

    If Not Page.IsPostBack Then
        GridView1.DataSource = source
        GridView1.DataBind
    End If

End Sub

Protected Sub GridView1_PageIndexChanging(ByVal sender As Object, ByVal e As GridViewPageEventArgs)
    GridView1.DataSource = source
    GridView1.PageIndex = e.NewPageIndex
    GridView1.DataBind
End Sub