Actually I have Multiple update panels on page, which update different values on server but the problem is that I have textbox to which I attach javascript class for datepicker on Load event.
But There are other updatepanels before that date TextBox, when I update them first calender image with date control which is in updatepanel disappears. or it remove the calender which is next to the textbox.
txtDate.Attributes.Add("class", "show-week w16em dateformat-d-sl-m-sl-Y");
Before using any updatepanel its like this

After we upfdate any updatepanel control its like this.

But the Date controls which are not in any updatepanel are ok and working.
And in these updatepanels I actually saving values in DataTables and save these DataTables in viewState …. no change in HTML.
I guess the datepicker and its textbox both are placed in an updatepanel?
If you do an ajax-postback with UpdatePanels the server will send html back for all UpdatePanels on the page, and refresh all of their contents, so your datepicker will disappear.
If you set
UpdateMode=Conditionalon the UpdatePanels then only the UpdatePanels that caused the postback will update.Another option is to recreate the datepicker with ScriptManager.RegisterStartupScript, but this possibly resets its value (depends on which datepicker it is).