From time to time, we have noticed that Google Apps users that also use the Google Apps Outlook Sync tool to keep their Microsoft Outlook synchronized with Google Apps may experience data that randomly disappears from Outlook, but remains online in Google Apps. This can include email messages, calendar events and even contacts. The exact cause is generally related to Outlook and the way it writes data to the Outlook PST file. Google has added a "Re-sync" feature into the Outlook Sync Tool that you can use to re-sync and download your Google Apps data to Outlook and fix the problem of missing data.
You can view the support article with detailed information on how to perform a Re-sync of your Google Apps data here: http://mail.google.com/support/bin/answer.py?answer=184933