This line is your problem.
Temptab = New AjaxControlToolkit.TabPanel()
You reinitialise the control with each click of the button. This clears the tabs.
The tab control should be constructed and included within the main page load.
So add the tab control and initialise it during the page load event.
You can manage if it's rendered using the
Visibility
property.
This will register the control properly within the ASP.Net event architecture.
Then when the page posts back and reloads. ASP.Net engine can use the
ViewState
to manage and reconstruct the previous state of the control.
Leaving you only needing to make it visible and add the tab in your button event.