Dear Saurabh,
When close button of the NTabControl is clicked the active NTabPage is removed from TabPages collection.
You can reopen this tab page as you add it to this collection again.
nTabControl1.TabPages.Add(nTabPage1);
or
nTabControl1.TabPages.Insert(0, nTabPage1);
nTabControl1.SelectedTab = nTabPage1;
Could you please elaborate the second part of your question, because I am not sure I got it correctly.
Regards,
Angel.