when i click on button all controls become disappear

I have number of  buttons on my page. i want 10 button per page and then need next button for loading next 10 button. when i click on next button next ten button load but when i click on any button for loading page all button become disappear . how can i solve this problem. 
public partial class _Default : System.Web.UI.Page {
int j=1; 
int k = 10; protected void Page_Load(object sender, EventArgs e) { if (!IsPostBack) mypage(); } private void mypage(){
for (int i = j; i < k; i++) { Button b = new Button(); b.Text = (i + 1).ToString(); b.CommandArgument = (i + 1).ToString(); b.ID = "Button_" + (i + 1).ToString(); b.Click += new EventHandler(this.b_click); //pp.Controls.Add(b); PanelProductButton.Controls.Add(b);
} }
private void b_click(object sender, EventArgs e) { stri ...