This can be a symptom of JS versions conflict. Check that you don't have two versions of prototype script on the same page.
Thak you Nick for your reply. I have two prototype version in my application I have removed and tried now I got a different error. I am able to find reolve that I have a ecruiser datatable in my page. If I remove that component my suggestion box is working fine. So I just replaced that component with richfaces datatable.
Now every think is working fine.