Don't panic yet, Yifang. Apparently when you changed your code, Adsense was triggered to reassess the ads suitable for your content. When it was not able to associate your page to the original content, it reverted to PSA's. Give it a little time to match your content to the ads and they'll come back. Pages that have ads on them for a long time already are usually more immune to changes, since I think Adsense 'remembers' the ads associated with an 'old' page.