Drop Drop filled with AJAX Web Service Getting empty on Post Back.

Hi I am filling a Drop Down Control using AJAX WebService but i am facing a issue.

 Issue: Drop Down is getting filed propery with the AJAX WebService and JavaScript Code. But now when the page is getting loaded after a postback, the contents of the drop down are getting lost.

For Eg: I have a Drop Down and a button. The Drop Down is filled with AJAX WebService Code and on Button Click the page is doing postback and again  page is displayed. Now when the page loads back, the drop down which was filled with Ajax is empty.

 I tried few things but was not able to identify why the Drop Down is getting empty after post back.

 Please help me in resolving this issue. I need the resolution urgently.

 Thanks in advance for help.
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